One solution would be to not give basic income for children, but to give it to children once they are working age (~16) or when they become an independent adult

One solution would be to not give basic income for children, but to give it to children once they are working age (~16) or when they become an independent adult (18). Everyone gets a certain amount to provide a very basic life on, where they get to decide how their money is spent. If they rather spend their BI on children instead of some other area of life, then that is their choice. If they want both, then they should continue getting a job to acquire funds beyond the BI amount.