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There are several,but the first one to try should be whiteboard. A plain white surface,where you can choose from different pencil colours.
 
Just what the subject says..... Thanks.

You didn't specify the age, but I'd say Linea is pretty simple and yet fun app to draw. It's not intended for kids, but it's very easy to get into.

Also, Paper is similar in this way too.

Again, these are not made for kids, but I think kids can enjoy using them.
 
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