I resisted mud play for a whole year. Then one afternoon I gave up, handed over a tub of soil and a jug of water on the balcony, and got forty minutes of the deepest concentration I had seen all week.
Now the rule at our house is simple: mud lives outside, shoes come off at the door, and everything gets hosed at the end. Nobody cries about a stained t-shirt any more.
What they actually do with it is not what I expected. There is a lot of potion making, a lot of grinding petals into 'perfume', and a surprising amount of careful planting.
It is also the cheapest category on the whole site. Soil, water, and whatever is growing near the door.
Tip
Use bagged potting soil rather than random garden dirt for toddlers, and keep it away from anywhere pets use. A pestle and a bowl of petals turns plain mud play into 'potion lab' and adds another half hour.