The casting material remains flexible even when it’s set, so, all that’s needed is one very gentle pull and your baby’s foot is free (heel first).
Casting feet for the little one's is pretty challenging. They tend to lift their toes and arch the foot.
I normally deal with this by holding the ankle firmly, with my thumb on the top of the foot.
It gives a degree of control to keep the foot immersed. Once the moulding material starts to set, I will move the container with the foot.
If the baby is fussing, they usually do so in spurts of a few minutes. Wait until he /she takes a breath, and quickly remove the foot.
Ideally there should be no residue on the foot for a clean mould.