I think it is a good way to saving money for your child but you need to introduce bitcoin to your children if they are old enough to understand it. Bitcoin is a unstable investment for now, but, in the future, bitcoin will become popular and more valuable so that saving bitcoin is a good plan. However, i hope you will have a good introduction about bitcoin to your children.
For the children, you need to pay attention for their future. Future of the kids will be even harder because the world is getting more populated clearly. That's why you need to save btc for them.