Financial gift for a child

I'd like to gift my niece and nephew $500 in MU or XEQT so that it could grow with them. What's the best way to do this? They are 5 and 9.

I prefer not to do it in my name, or in their parents' names. I have absolutely nothing against their parents; I just know if I give them money and ask them to put it in the kids' names, they will say there is no need to as it is not a birthday or Christmas, and will refuse it.