Soundproofing is VERY technical stuff. Most of the time people talk about it and have no idea what they are talking about. The video above isn't bad, but it doesn't help much between floors. Virgil has the issues bang on though. With what you are describing, you definitely need to pay attention to both types of issues. How much money are you willing to throw at the problem? In most cases it isn't a huge deal, though I am particularly pained by noise so I appreciate spending real cash on it. Two layers of drywall with acoustical sealant between and a z-channel or resonant channel plus putty behind any penetrations will make the room very quiet. Basically increase the STC by 5-10, which is a huge change. https://www.youtube.com/watch?v=-IfYuNnRLgU Highly recommended. Plus since it isn't "firmly" attached, it can help lower the chances for random nail pops from house settling or whatever. Insulation in the space will help, but it is NOT a major reduction, unfortunately sound just doesn't work that way. :( Stop air movement, add resonant mass, and understanding the real limitations and goals. That's the way to do it. Oh and also concern yourself with the layout of your HVAC. You can go to all of this effort to reduce sound, only to punch a hole that directly connects between the two rooms via a big air vent. That has to be designed right as well. Maybe insulated lines, rubber disconnects (I don't know the right name, just something to allow the ends (all of them) to be soft connected vs hard) add in some bends etc.

I did just what you describe and I'm tickled pink with it. In my case it WAS a load-bearing wall, but that just meant I had to header off the opening. I created about a four-foot archway between the two sections of the (now one)room - making one section the bed area and one the sitting/dressing area. I can't tell you what it costs since I did it myself, but I really don't think it would be that bad, depending on what needs to be done. I had to move a electrical box and add another (to stay up to code) but it was mostly a framing/drywall job. I would say the most important thing is visualizing the feel/scale of the new room as well as new traffic patterns and usage patterns to come up with a good design. In may case I created a couple of new walk-in closets to keep the scale of the resulting room comfortable. I know I've been in several "de-partition" rooms that feel just like that - two rooms with a wall knocked down.

When we build, there won't be any rooms above our Master Bedroom. At the moment, my almost 13-year-old son and 3-year-old's room is over our Master Bedroom. My 8- and 10-year old's room is above the living room. Their bathroom is between their two rooms. When sleeping in, I can hear every single movement that my 13-year-old and 3-year do. There was carpet in the room which helps with the noise, but I hate carpet and put in wood flooring. If he drops something, I hear it. When he goes to the bathroom, I hear it. When all 4 of them are in one room, I can hear them. Just as this, I can hear every movement in my other boys' room when in the living room. They are very rambunctious and like to jump around. Another problem I have is that the pipe from their toilet/tub goes down the wall which is located between our living and kitchen/dining room. When they flush the toilet or let out their bath water, the pipe rattles in that wall. I know every time they use the bathroom and so do my guests. Most times it is rather embarrassing.

I like my first idea best as far as use of space. I know people say not to put toilets on an outside wall. We had one in our last house in Iowa, but we had 2X6 construction. I don't know how high your window is, but I'm guessing the toilet would fit under there. The shower would be 5 feet long or shorter if you want a wider space to get to the toilet. (kind of tight the way I have drawn). You could do a 5 and half foot long shower and do an angled door at the left end to allow room between the shower and toilet. The vanity would be about 7 feet long, unless you do a 60 inch vanity with a linen cabinet at the door. I would reverse entry door swing if you do that.. The second top right could work and you would still have a small closet. The bottom left plan would require waterproofing your front window. Hopefully someone will give you other ideas. The bottom right plan is bigger, but no closet. Each square equals 1 foot.