One thing to consider (and I hadn't really thought of it) is that you might be better off investing elsewhere.
As much as we love our kids and with the best intentions in the world they *may* not turn out quite as we'd like. Then consider that on their 18th birthday THEY have control of whatever money you've paid into that account. Not you, them!
So, you spend the next 18 years investing for their future and they blow it all on day one on a motorbike!
Probably better to look at an ISA for them.