The point is though it shouldn't cost you any money to go to work.
Companies need to be making it easier for women to remain in work and not penalise for expensive and difficult to access childcare.
If you were a man, it would be less considered because there is an inherent belief that a woman is around somewhere to manage the kids. Unpaid work.
But here. The work needs to be paid for, and the company should bare that bill if they want you to travel.
No contract will ever stipulate this detail. But you won't change policy if you don't challenge and ask.
Any one here saying it shouldn't be paid, try to imagine how helpful it could be for you if your work offered similar.
Alternatively organisations can pay more in the first place, especially to to women, the gender pay gap, to alleviate the issue at all.
This isn't on you. It's on them. Challenge them.