I'm no custard snob but remember that custard powder usually has yellow colouring in it, so if your child is sensitive to additives, it might not be a great idea .

I just heat some milk, vanilla bean, and raw sugar, then whisk in some egg yolk. It's always kind of a rush job, and I don't have a recipe I use, just guess quantities. So if it doesn't thicken as much as I want it to, then I add some cornflour to help it along (after it has been mixed with milk of course).

Sorry, that's very vague I'm sure taste dot com would have a recipe you could try

ETA Oh, it's not at all hard! Might take you a couple of goes to get comfortable doing it, that's all, and it's best done with a whisk. And remember that it will thicken a little after being refrigerated.