I pick up any 'chunks' of food off the floor after each snack/meal, and I sweep the kitchen floor after their last snack of the day. I mop once a week.

Two things I would suggest. If you don't already have them, get the high chairs/boosters with the trays--it keeps food much more contained than having a booster without a tray that just pulls up to the table. Second, when the kids are done eating, take their food away! LOL I find that my kids will eat what they want within the first 10-15 minutes, and then they start playing with it. And that's where most of the mess happens.