Stocks ofcourse. They are young, the stocks are down and the amount you are talking about is very small.
Let them have some cash in hand to learn about prices in stores, giving change, wants/needs, and overall cost of things. Can you imagine when they discovering that you bought them 10 apple shares when they were very young and now, at 8-10, they can see the price and how much it has grown if it has. If it hasn't, still a learning moment. Buy a few things and see which has grown the most and why.
Always amazes me that people don't use money to learn about money. Instead, they stick it into some far away account to sit and rot away so they can tell the world how their kid has savings account. 1980s is calling?
Again, use money to learn about money.You could DCA $50 for them every month til they are 10. Then talk how you all got there and how the things you bought fared.