How do I balance letting kids have outdoor messy play with not making my garden look like it hosted the Battle of Agincourt?

I've recently made the kids (9,4,2) an extended mud kitchen, a wooden playhouse and a little picnic table area. In my head they'd all make cute little mud pies with flowers and do lots of quiet play in the playhouse. In reality there is mud everywhere and the whole backgarden looks like it's hosted french and english troops for a tiffle. 4 year old learnt about mudhuts in a book and was convinced she could recreate one with the playhouse so spent an hour smearing mud on there. 2 year old has discovered outdoor water tap and does 45 runs with a little beaker. Do I just leave them to it? Do you lot just let kids do what they want in the garden or is it more structured play? At home we have pretty firm rules of tidying and keeping the area clean but I have no clue about what boundaries to set in the garden. At the moment my tactic is just look my recliner chair with a cup of tea like I'm watch my own little medieval farm animals before running a bath and scrubbing them all down.