the children already started == as crianças já começaram
the children started already == as crianças começaram já
can you say both?
Yes. The first one is more frequently used though
Why was filhos correct two questions ago but now they want crianças?
I think "os 'filhos' já começaram" doesn't work. It's incomplete. "Os filhos de quem?", "começaram o quê?". I think "as 'crianças' já começaram" is better.
As should have been able to be applied to meninas, as well