In my case it was a large, drunk adult. In case I get falsely accused of having my seat reclined based on some wild assumptions, as well as being rudely told 'what nonsense' and to 'get real' as if I were imagining things, I should perhaps clarify that I had my seat fully upright at the time. Come to think of it, it probably gave the drunk guy extra space to gather more momentum.

Just saying that it can happen and that the OP's comment about the neck pain caused by a kick on the seat cannot necessarily be dismissed outright as a fib or exaggeration as has been done by some, without taking position in support of the OP or otherwise (although I do think the OP has over-reacted given the circumstance, although in my case I should have reported the drunk to cabin crew instead of saying nothing, perhaps.)

If hit on the upper part of the seat, it can move a fair bit and can cause your neck to jolt and cause issues (especially if you are prone to neck issues in the first place). Although it was a large adult in my case, I can see it can happen even if it was done by a child.

I prefer not to jump to a conclusion that it was some kind of a nonsense and give most situations at least some benefit of the doubt even if I do not agree with the position taken by the poster.