I would double it if possible. if she digs a hole 4 inches tall, theres only a 3 inch roof, and it will probably fall in on her. Everyone has what they like to use. I just hate seeing them put a ton of work into digging just to have it fall in on them, and regular old fashion dirt seems to hold its shape the best. If you do a mixture, I'd go easy on the sand part. Having enough moisture in there helps it hold to. You dont want it muddy or anything, but it shouldn't be dry either. I shoot for what I think will stay together, like where you can make a dirt snowball with it.