I have a 2yo son and another on the way. I built two AR15s of all top-shelf parts because they are what I like to shoot and because they may not be easily (legally) available one day. My children will each inherit one, legal or not. My children will of course inherit te rest of my firearms as well.
If you don't have an AR15 in your collection, you can buy or build a fine one in almost any caliber you like for $2k. 300blk, 458socom, 50beowolf all do great flinging cast boolits.