In Chapter 954, we see the Numbers enter Wano, and the children are legit afraid of getting eaten by them.
In the Chapter 999, we see that Ace and his companions are rescuing the abducted children from Onigashima.
So far in Wano, we haven’t really seen Kaido having children as his slaves or workers. I think that’s because he doesn’t find them strong or useful enough for his force.
So I found it odd that why we would see children being kidnapped in Onigashima. I think those children were there as food for the Numbers.
Once Ace found out what happened to the children from the Wano villagers, he went to Onigashima to rescue kids and fight Kaido for his doings. But as fate have it, he met Yamato instead.
*Theory by s9973