Look, the airplane is a giant aluminum tube stuffed full of as many people as they can cram in.
Sometimes, we all have to "take one for the team" so to speak. Especially when it comes to little ones. Just like riding on a city bus - sometimes you're stuck with the seat over the wheel well, and your feet get cramped up. Just how it is.
Thankfully, the human psyche offers an innate protective instinct towards children - so we tend to accept things like this without complaint because it's for the protection of a defenseless child.
I know you're not complaining per se, but there is no guarantee of reclination. I'll be honest - I don't find the recline very comfortable anyway. In fact, I prefer choosing Row 20 on most flights since I never recline my economy seats anyway - and the row in front of me usually can't recline either. Best of both worlds.
So I ask - please be careful what you complain about. Approaching it as "something you paid for" -- you just might inspire United to assign a value to it... and give them another monetization idea - "Swipe your card to unlock your seat's recline button now!" Clearly the offer of FF miles was a nice gesture on United's part... I'd just let sleeping does lie, lest we open Pandora's box here.