You are hiding assets, that will make it illegal no matter where you stall it. Meaning a gift will probably be void. Not sure about Thai law, but most countries have laws against that.
Incase it is allowed: Money gifted to your child will mean it is theirs and the parents manage the funds accordingly. Menaing you can not use it for yourself, but can use it to pay medical bills and edcation for the child.
A court will grant joined custody over the child, so you would most likely have to consult the mother when you want to spent it.
Like scorecard says, best option is a fixed account that cannot be touced till the child becomes an adult.