Is it permissible for boys and girls aged between four and nine years old to wear trousers while performing ‘Umrah?
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If a child enters the state of Ihraam (ritual consecration) independently, in case he is of discerning age, or if the intention of entering it is made on his behalf by his guardian if the child is less in age than that, then all that applies to an adult concerning the prohibitions of Ihraam would apply to him as well.
Thus, a boy in the state of Ihraam, may not wear form-fitting cloths such as trousers and a girl may not wear Niqaab or gloves.
As for girls wearing trousers beneath the clothes other than in a state of Ihraam, or by a little girl who would not arouse males sexually by wearing them, then there is no problem in wearing them. If she is grown up and wearing them may arouse males sexually, then she should not wear them except beneath her clothes because wearing it externally would reveal the details of her body, display her beauty and draw people's sight toward her, which would incur sin on their part.
Allah Knows best.