I would have thought a trust fund would be best if this is for university, if for regular schooling then it would have to be in a normal account. A trust fund or child's bank account can be paid into internationally.

Maybe the best bet would be an ISA in your cousin's name: your aunt could give her the money to put into that, up to £7000 a year at current rates. That is tax-free savings for adults and can be accessed with minimal notice if you get the right account.