Lots of kids are messy whilst on holiday. The issue is whether the parents then clean up after them to leave the holiday let in good condition or leave it in a state.
People I know who have holiday lets tend to charge a damage/cleaning deposit because more than once, they have found the filth on arrival is far more than could be expected. You are asked to leave the holiday let clean and tidy, ready for the cleaners to do a quick whizz round usually. If you’re there for a couple of months and it’s not just a week of holiday, the contract will have specified a decent standard of cleaning. Finding bits of food is clearly not at that level by any imagination stretch.
Many rentals require the property is cleaned to professional standards on vacation. This doesn’t mean there has to have been a paid cleaner but it has been cleaned to that level. That means grease has been removed from cookers and around cookers and in oven hoods, carpets shampooed if that’s needed, curtains cleaned. Wear and tear is accepted, but dirt isn’t.
Op, if the owner then had to pay someone for several hours cleaning to get the property up to scratch for the next tenant, then your cleaning wasn’t good enough. I’m surprised you weren’t charged.
Most small kids are messy and drop food. Eating at the table, having a plastic sheet down and certainly sweeping up after each meal by the adult is what is needed. It’s not the kids that’s the problem for other adults, it’s the adults not clearing up after them. You see it in restaurants sometimes. Restaurants expect small children to have made some mess and that they will need to clear up a bit. Parents who leave food. All over the floor and debris spread across the table and not even piled onto the plates, are in my view pretty filthy. Honestly, who leaves used wipes just lying in the middle of the table or used cutlery strewn everywhere and drips of food smeared everywhere? Wouldn’t you just pile it all onto a plate and use a napkin to wipe where a small child had dropped a huge splodge of wet food?