Ref. No. 39 / 810
In the name of Allah the most Gracious the most Merciful
The answer to your question is as follows:
After separation, the mother shall have the right to nourish her son up to seven years of his age and her daughter till she has reached the age of puberty, while father would be responsible for their all kinds of expenses. When the boy has reached the age of seven and the daughter the age of puberty then the right of nourishment shall turn to the father. Hence, after the said period, father has right to take the children into his custody.
And Allah Knows best
Darul Uloom Waqf Deoband