How do I make a “Frozen “ castle big enough for a chid to play in

You can make a castle out of cardboard and use the Styrofoam for ice blocks. Since they are already white, all you have to do is add glitter.

https://www.bing.com/images/search?q=diy+frozen+castle+cardboard&qpvt=DIY+Frozen+castle+cardboard&FORM=IGRE

I would start with a large appliance box and add styrofoam to decorate it.

Here's a cardboard idea! Fun!

Check with stores that sell appliances and ask for a couple of boxes. That would be a good frame. Then add styrofoam, paint, glitter, and fake snow. You might still be able to pick up some icicle decorations in after Christmas sales. I would find a few inspiration pictures to use as a pattern.

A large furniture box maybe? Not sure how old the child is.

This DIY looks adorable. https://www.hallmarkchannel.com/home-and-family/how-to/diy-frozen-castle-out-of-cardboard

Check out the castle at the bottom of this link using cellophane wrap!

http://www.playathomemomllc.com/2014/12/stained-glass-house/