A bit ridiculous this. Having cheffed/managed a restaurant, a few basics:
1 - A parent has every right to take their kids out to eat at a restaurant, unless the restaurant explicitly states that children are not welcome.
2 - Kids will eat messily at certain times and certain ages.
3 - The restaurant has the right to expect the parents to keep a reasonable degree of control over their children in terms of discipline, boundaries etc. and make sure the other diners aren't unreasonably disturbed. As long as the mess isn't deliberate (food fight!) then it doesn't qualify as bad behaviour.
4 - A restaurant is part of the HOSPITALITY industry. Sorry if this sounds like I'm being up myself, but every member of staff there is paid to be hospitable. Their job is to ensure that you enjoy a meal without some of the hassle you have at home (cooking, washing up, CLEANING etc.) The cost of cleaning, wastage etc is factored in to the meal prices at any restaurant. If it isn't, then the manager is simply a bit shit at his job.
5 - Almost more appalling is the cowardice of the manager in only mentioning these things to you when he knew you were about to leave. If he had an issue then he should have told you immediately, giving you the option of either cleaning as you went along or going somewhere else. Guessing he didn't want to lose the business.
YANBU.