In America, clay traditionally has been used for the 3-5 age group but the educators in Reggio Emilia changed this with their work with infants and toddlers. As a result, more and more centers are giving clay to infants and toddlers to explore. The purpose of giving clay to these age groups is to give them the experience of the material at an early age to develop a relationship with it. What I learned from Reggio Emilia was that the after having extensive use with the clay from an early age, the three to five year olds were then able to manipulate and use it at a much higher level - to use the material to express their ideas. In America we have substituted play-dough for clay especially with our youngest children. Play-dough is easy to make, doesn’t require water and is edible. However play-dough is a cooking material not an art material. I believe it is completely appropriate to let infants and toddlers explore play-dough - especially the homemade kind when it is warm to the touch. However clay holds so much more potential.
I would recommend giving infants clay in large blocks on the ground on a large mat or plastic surface. See what they do with it. Encourage them to push it, touch it, tear pieces off. You could also cut large slabs from the whole block and let them build with it. If you are using Magic Mud, it is not toxic and edible. So, if infants put it in their mouths, it is pretty much the same as eating sand or dirt. Not great but it won’t hurt them. Some educators have utilized pacifiers when having infants work with art materials to help them not mouth the material. This depends on individual philosophies around pacifier use.
In regards to toddlers, you can start of with a large block or slabs and transition to small amounts of material. Toddlers love to explore - the material is the motivation but I would encourage teachers to find creative ways of introducing it to them to keep their interest alive - perhaps one day present the clay in big balls, another day in long coils or snakes, etc… Constantly reuse it.
At some point in the year (But not too early in their exploration), it is wonderful to save their marks - teachers can create slabs and let the little ones press their fingers into it or some natural or made-made textures. Teachers can then poke a hole in the top of the slab and then fire them to preserve the marks. It makes a great take home gift. If you don’t have a kiln, Magic Mud can be brittle when made into a slab but if the dried slab is painted with a few coats of Elmer’s Glue or Acrylic sealant, it should hold up.