Correct word for a little toy that always stands up?

I'd call that a Weeble Wobble or Weeble but I think that's a trademark usage of a roly-poly toy (as GileBrt notes as the generic term for one) much like Hoover is not only used for vacuum cleaners that are made by Hoover but other manufacturers too. Whether this is a UK convention though I'm unsure [Judging from comments it's AmE too].

I'm also certain that somewhere I've seen a cartoon or kids TV character roly-poly toy that even says "Weeble Wobble" as it bobs back and forth. (Also Weebl)

I believe it is called a roly-poly toy. :)

Wikipedia

A roly-poly toy, round-bottomed doll, tilting doll, tumbler or wobbly man is a round-bottomed toy, usually egg-shaped, that tends to right itself when pushed at an angle, and does this in seeming contradiction to the force of gravity.