childrenは複数の子供達だからです。
例えば、
I do yoga.
He does yoga.
They do yoga.
children はtheyです。(複数の人たち)
1.When do your children do their homework?
→When do they do homework?
2.When does your father cook dinner?
→When does he cook?
動詞の最後にsがつくのが、三人称単数(he, she, it, your father等)のときです。