As folks have mentioned, aging for 21 years is a crapshoot. I like the idea, and entertained it myself when my first kid was coming along. Let me suggest a few other ideas:
Target a younger age than 21. You can serve your own kid in your own house legally in most states (be sure to check). Maybe 13 or 14 would be a good “starting to be mature enough to drink at home supervised” threshold, and sharing a bottle with them then might be reasonable.
Wait 5 years or so and get them to help brew something to drink in another 8 or 10 years, instead of aiming for all of the years from the outset. Even if their participation is minimal - dropping things into the kettle, maybe, or having their hands on yours while capping bottles - it’ll be something they participated in.
While you’re at it, make sure you brew something for your wife to drink relatively soon after the kid is born, assuming she liked beer before.
I think it would help to store the beer cold if that’s possible. Maybe, stash a few in the back of the refrigerator and enjoy the rest once a year to see how it’s progressing.
One thought I’ve had over the years is to bottle condition, then store the bottles in a purged/sealed keg to minimize O2 ingress. I don’t know how much shelf life that will buy you, but it might be a decent insurance policy.
I had a similar idea – daughter was just born. Perhaps I will purchase a barrel of bourbon from a small Kentucky distillery and wait to bottle until she’s 21.