Good old fashioned plaster of paris (without the bandages!lol.) will do the job. Or decorating filler, the powder sort made up really runny.
First you make a cardboard 'surround'. In other words a cardboard tube around the foot prints. Then make up plaster or filler to a easily pourable liquid, tap to remove bubbles before you pour. Fill card mould & wait until set. Lift plaster off the ground carefully when fully hardened & clean off and debris using a soft brush, removing the card surround. Show to kids!
You will know if the mix is liquid enough when you get full print impressions without air bubles.