There's no need at all for any brusqueness or a set of demands though - firm and cheerful is my motto, and also a lot of catching someone being good. There are no immoderate emotions involved either, or expressions of aggravation that are immoderate. Mary Poppins is a bit of a caricature, but something along those lines is what I aimed at.
If the DCs seemed to be spilling food just for the fun of it and they had been there for a bit and eaten something, lunch would be over if they kept it up after being asked if they were hungry any more, with the comment, 'Ok, you're not hungry, let's clean up so' - and then we would clean up together, hopefully with the opportunity for me to give a compliment on the co-operative way the DCs picked up the broccoli bits, or how neatly they wielded the paper towel. (I don't like to use cleaning up as a punishment because it makes it harder to secure help when they get older if they think it's a punishment or a drag, or there's some blame involved.)
Again, with messes, some of it is just curiosity about how the food looks down there on the floor or wondering can water be thrown and look the same when it lands. If you take it very earnestly and explain at length why the DC is not to do it, the child learns that there's a certain amount of attention to be gained too, along with the satisfaction of curiosity.
I think it's just as important to praise specific positive behaviour as it is to name the specific negative ones, so no 'You're naughty' or 'You're good', but instead, 'That was messy' or 'You co-operated nicely'.
I think it benefits children enormously to have choices in their lives (within reason, what to wear, what activity to do, whether they want to play with a certain playmate or not) but I also think boundaries are important, and since I personally don't like a wet and slippy bathroom floor or to end up with soaked clothes, that is a line I drew, firmly and with lots of paper towels.
There's a middle ground between cross and angry (and downright rude) on the one hand and explaining everything on the other - and it's not the opposite of reasonableness or moderate emotions or at least their expression - you can be firm with a smile. We clean up the messes and move on with our lives and nobody gets hurt.