As far as location goes, this could be anywhere there is a good clean surface in the house that would be out of the way. This could be a spot in the basement or the attic, even the kitchen table. If so desired, a child could also have a spot in the garage, or outside on the porch or even in the yard to do their slime-making.
It is important, especially in an indoor spot, to always make sure that the table, floor or furniture is properly protected, using a tarp or newspapers. When the spot is determined, it is a good idea that the child wears an apron or a painting smock over their clothes, and even plastic gloves since the process could be messy.