I was thinking about setting up a trust fund for my girlfriend’s niece to get her off to a good start when the time comes. I’ve checked a bit and it seems the only way is for her parents or guardian to control a savings account. I specifically want a trust account cause I don’t trust her parents to fare well with that account, and better she’s in full control when she turns 18. 
Have anyone else done something similar in Thailand? Any banks who offer a similar service or account as a trust account where the child is the custodian but cannot withdraw until a certain age?

I don't think personal trust accounts are possible in Thailand.

 

Perhaps the closest you can come to that is a fixed term annuity with your GF's niece as the beneficiary.  I'm not well informed on annuities so my suggestion should only be used as a basis for inquiry to a better source.