How do you make a mold from kids' handprints in a concrete slab that can't be moved?

Clean the concrete very well. Make a small wooden frame around the hand print (1 ft sq) should be big enough. Spray the concrete with non-stick frying pan spray (or other release agent). Mix plaster or mortar and pour into frame. Let set. Lift off hardened material and remove frame. This can be used as a "negative" to make other prints.