But the main suggestion I'd make: Buy her a dress to get messy in. Shows that you care enough about her not to risk her own gear, and you can have fun going out shopping together to get her outfit for the messing. Charity shops are awesome for really nice outfits at knock down prices, but supermarkets snd high-street fast-fashion shops like Primark are also good places to buy sploshwear.
A very general rule of thumb that works most of the time. . .
Natural Fibers: AVOID! If you use cotton, linen, wool, etc. the fabric will soak up whatever mess is on it and will stain. It may be hard or even impossible to wash out.
Artificial Fibers: Nylon, Rayon, Spandex, etc. don't absorb liquids and colors so USUALLY any mess can easily be washed out.
Sugar based messes come out easier than fat-based messes. So lard, gravy, whipped cream etc. might stain while syrups, marshmallow goo, will be less likely to.
screen_name said: Also get it in the washing machine ASAP- much much less likely to stain if it doesn't dry in.
^^^^ THIS, seconded.
Also, do two washes.
For the first wash, set the lowest temperature you can, no more than 30 degrees C, use double the usual amount of detergent (we use liquitabs, dungeon washes get 2 of them), turn off the spin, and turn on as many extra rinses as the machine can do. So at the end of this first wash the clothes are soaking wet but have been thoroughly washed and rinsed.
Then pop in another load of detergent (just a standard dose this time) and now set the temp to 40 degrees, turn the spin back on, and just have one or two rinses at the end.
That will get most messes out of most things.
Note if washing leather shoes or boots in the machine, use cold water and minimal detergent, it's more the wash action that gets these clean. And no spin, it'll destroy most non-trainer footwear, won't get them any dryer and may damage the machine. Take them out and hang / prop up to dry - boots placed upside down over freestanding broom handles are a good way to dry them.