It's tricky as any contract entered into by a minor is not enforceable. You'll find the same in UK so few places want to deal with minors.
I believe Degiro have a minor account facility. It's a joint account, in the name of child but with you as legal representative so contract can be enforced. There may be others offering similar.
Look into the tax treatment as well, particularly if the original source of their funds was a gift from you. I haven't looked at it but there might be anti-avoidance rules to prevent parents avoiding tax by buying assets in name of children.
Alternatively, you could buy them in trust for them, but it would be taxed in your name until transferred to their name at 18.