The kinds such as yew, holly or box suitable for making a nice hedge beneath the other trees will never get there before you leave. Bamboos are quick but not cheap. And there will be no really cheap way to plant 175'. What about erecting some screening just near the house, around the play and sitting area instead of trying to put up another Great Wall?

Would Giant Green Arbs work if we kept them trimmed? This area maybe gets about 2 hrs of sun/day

"Would Giant Green Arbs work if we kept them trimmed? This area maybe gets about 2 hrs of sun/day"

Sure, they'll grow but you'll find them rather sparse.

No cheap solutions for 175'. Do you really need to cover the whole 175'? What are the potential sight lines? Do the areas of your yard (or their yard) that see less frequent use require the same level of sight line privacy as the heavily used areas?

If you really want this privacy screen, you start by taking down the pines. The you plant something else.

What grows under large pines in the wild is pine needles. That's it. It is a very difficult environment for plants, particularly anything that isn't a ground cover.

"What grows under large pines in the wild is pine needles. That's it. It is a very difficult environment for plants, particularly anything that isn't a ground cover."

This is a particularly inaccurate assessment.

The understory of a pine forest is, more often than not, particularly rich in flora. Pines are an essential successional stage in the natural development of eastern hardwood forests. They do so by enriching the soil, providing shelter from weather extremes, and providing partial shade. What the OP is describing typifies the dappled partial shade seen in a pine woodland and need not be seen as an obstacle.

And this flora is ....?

Seriously, around here any natural pine forest has a remarkably barren forest floor. Even the spring ephemerals that grow in the heavy deciduous forest don't exist.

White pine? Natural or plantation/reforested? Age, canopy height? A younger, denser stand will tend to remain a mono-culture until the trees age, elevate their canopy, and go through some natural attrition

OP is posting from zone 7....probably not white pine but a hard pine which casts less shade. OP also points out there is about 2 hours of full sun each day and the low branches start at 20', so more mature trees.

You can tell that these pines were planted on purpose since they are in a direct line. However, they are 100'+ in the air, so I'm afraid taking them down will result in a bigger feat than we are up for....
Just thought that there would be an "easier" solution.
Yews won't work?

White and red pine. Some groves are definitely natural, though second growth. Reforestation isn't something that is done much around here since things tend to grow back on their own. Age and canopy height vary from 40+ years to 20+ ft, though the red pine forests I'm familiar with are definitely younger than some of the white pine ones. Some of the white pines have trunk diameters more than 3 ft across. Those are the places where absolutely nothing else grows - the big, old white pine groves.

No, yews would survive, but not grow at all quickly or thickly.

If I did a double staggered layer of Giant Green Arbs, would you think that might look ok?

You all are SO helpful! I have no clue at all!! ;)
I like the look of the GG Arb hedge....

The lady from the local nursery also recommended we plant more pines, but I was afraid that they wouldn't give privacy?

Ok....I went a tangent regarding pine forests.

Yews are probably one of your best choices....look for T. x media 'Capitata' or 'Hicksii'.
If you have really hot summers, Plum Yew might be better (Cephalotaxus harringtonia) but is harder to find and more expensive.
If hemlock adelgid (insect pest) is not a problem, a clipped hedge of Hemlock (Tsuga canadensis) is great.

Other possibilities include:
Japanese Cleyera
Common Boxwood
Various hollies (Ilex)
Manhattan Euonymus
Evergreen Privet
Cherry laurel
Fraser photinia
Various evergreen Barberry

WOW!

Awesome!!
Thanks for all the suggestions and help!
Seriously, this is incredible!!

Many hugs!

Smivies comment, "What are the potential sight lines? Do the areas of your yard (or their yard) that see less frequent use require the same level of sight line privacy as the heavily used areas?" is right on the money. You may be able to plant groupings of shrubs to give yourselves a good amount of privacy without having to plant a 175' line of shrubs. Clumps might look more attractive, too.

Your plants will do best if you can plant them somewhat away from the pine trunks as they will get more light. Plan on regular, slow & deep irrigation to improve the speed that your choice of shrubs grow, and plan to irrigate under the whole area of the pines, not just along the row of shrubs so that there is less competition for moisture.

A couple of words of warning:
-Yews will only be a wise choice if you don't have deer. They are a favorite of deer and will be nibbled to stubs, even if the deer eat nothing else. (They are one of only two plants I have had deer damage on since I live in an area where there is lots for deer to eat away from houses. My yews were bare branches and grew no larger until I ripped them out after a few years.)
-A few of the plants listed by Smivies aren't reasonable (and in some states illlegal) to use in some east coast states (barberry/Berberis, privet/Ligustrum) due to their invasiveness from seed carried by birds becoming pests in woodland areas.

Another option to think about would be to add some of the old-fashioned larger, faster growing rhododendrons like 'Roseum elegans' or even some of the PJM series rhodies which can often be bought relatively inexpensively at big box stores. I wouldn't use them as a hedge, but in clumps to protect sight lines they would be fine and since they are evergreen will provide year-round protection. They will bloom less than in more sun, but may well bloom some as they are part-shade tolerant. Plan to mulch these to help keep moisture even since they are shallow-rooted.