A nie dwudziestu?
dwudziestu is for male people
dwadzieścioro is for neuter nouns referring to people (so mostly kids- dzieci, and babies- niemowlęta)
and for mixed gender groups
dwadzieścia is for female only people, or not persons (all genders)
My answer, with 'dwadzieścia' instead of 'dwadzieścioro', was accepted (presumably wrongly).
"Dwadzieścia" does appear in the list of accepted answers..... I guess children can also fall into the "not-persons" classification :-) .
Frankly, I still need to make sure :D "dwadzieścioro" is surely correct, I need to check if "dwadzieścia" is acceptable or not...
"dwadzieścia" is IMHO not super clean Polish but 20 times out of 25 random Polish person will say "dwadzieścia dzieci" - especially in Warsaw ;-) Language evolves toward laziness. "dwadzieścioro" to my ears sounds a bit dated.