hobbyking asia will ship lipos but only via courier now.
Hand guns are ok but honestly, I've never pulled one at a field, only in cqb to clear small rooms where longer guns are a handicap. They are fun to plink at soda cans though
At the cost though, hand guns are probably the last part of your kit you're going to buy. Pretty much all the other gear has more priority, especially for outdoor.
My M4 CQB is only 1" longer than my MP5 from pistol grip to tip of flash hider. That's not going to make much difference with either of your kids.
Scopes are almost useless at the ranges we play at, you're probably only going to be accurate with a stock gun to 150' - 180' so having a 4x scope won't help much and that's on a perfectly calm day with no wind. I have a 1-4x on one of my guns but it's more for spotting than anything else.