If you can get, and keep, the crepe myrtle to a single trunk, it won't be as wide as a multi-trunked tree. The flowering branches do extend pretty far out, and can hang down/droop gracefully as the blooms open, possibly as wide as tall, but I haven't measured any big ones that were single trunk. That's why I suggested planting further from the sidewalk, so it doesn't block the walk. You could trim any branches that overhung the walkway back to the trunk, although the CM as a whole might look lop-sided. I am not sure you WANT to prune a 20 foot tree to keep it lower, unless you have access to a cherry picker, as it's rather hard to be safe that high up and reaching to prune? (if you are an aerialist, line-man, or professional arborist, then never mind) And CM's don't look good pruned shorter, IMO. Look up "crepe murder" on this forum or Google for it, and find photos of each, and you can see the differences in "murdered" treees and trees left to grow naturally. Trimming for shape is one thing, trimming for height is much harder to do and keep the tree looking natural. Try to find a CM cultivar that won't get taller than you want the ultimate height to be, and be aware that it will take some years for it to grow to that height. While I love lamb's ears, unless you get a non-flowering variety, be aware that it might not be the best for next to a public walkway. The bumble bees LOVE the flowers, and while you do have to work pretty hard to get a bumble bee to sting, it might upset the neighbors and passers-by to hear and see them buzzing happily. Not many people are aware of the differences between bb's and honey bees, and can react badly to any buzzing, bee-like insect.

I'll bet you added that plastic border fairly recently. Either that or you cut a root while weeding. The baby formed from a severed root of the larger crape. the root system of the baby tree probably mainly radiates out away from the parent tree, so you can keep that in mind when you go to dig it up. I'd try to get at least a foot of root length (based on my guesstimate of size from the picture and assuming root structure similar to what I'd expect in this situation). Crape myrtle root divisions are pretty easy to transplant. Just take your time and get a reasonable amount of root as you remove your baby crape. You may also want to be careful not to cut a lot more of the parent tree's roots in the process of digging up the baby. I'd give this job a rating of easy. P.S. Be careful to get down deep enough to get a good piece of root. Depending on various factors, the bulk of the root system on some of these can be deeper than what you might expect from a seedling of similar size.

I also don't think that it should have been put in water. The best solution would have been for the donor to wrap the roots in wet paper towels or newspaper, then placed the whole thing in an open plastic bag. This sort of treatment (not the drowning part) might have been ok if it had been the winter, but not with the seedling in full foliage. It only takes a very little time for the tender roots to dry out after being exposed to the air, and just as little time for them to drown if placed in water. Truly, visit your local garden centers, big box stores, and maybe even the grocery stores! You can find small crapes for very little money.

I've used to strike a lot of crepe myrtles & had good luck with both spring & fall slips. Rooted under a simple mister ("porch cooler" from Lowe's) in full sun. I treated them like rose cuttings--slightly wounding through the bark on the bottom 1" or so, used a coarse media of blasting sand #3 & coir. Perhaps the biggest difference is I always used a strong commercial hormone--Rhizopon #3--.8% IBA. It makes a big improvement for me when I try to root woody plants like roses & crepe myrtles. Hope this is useful.