T., you've gotten some pretty good comments here. Obviously, your first priority is your children's health. The suggestion to bring jars of juniors baby food for the toddler is A-1. You also must make sure that your in-laws understand your children are not to be fed from the table.
Although their constitutions may have adapted to the caveman stage where your in-laws can safely eat marginally tainted food, or worse (I have friends in Vladivostok, Russia who eat raw bacon with no ill effect. And it is common practice there!), your children's digestive systems are not strong enough to tolerate such extreme dietary fare.
Secondly, will you be staying with the in-laws during your visit? If so, why not volunteer to help with the cooking? In fact, go out and buy the main course entree. Have your husband's sibs agree to bring some side dish and tell Mom and Dad that this year, the kids are preparing the Christmas dinner for them. Make it sound as though it is a special treat and a gift for them from the group. Take an extra effort to keep the kitchen clean and the food that requires it safely refrigerated.
Thirdly, what is your husband's stand on the issue? Before the problem can be remedied, the in-laws need to understand that there is, in fact, a problem. They need to be told why you don't want your older child to be eating from their plates either and this needs to come from your husband. (Unless you have a very strong relationship with them, they are likely to view such comments from you as meddling, intrusive, and 'better-than-you' snobbism.) Don't be surprised, however, if they still take offense at the suggestion that their kitchen should be condemned by the Board of Health.
Remember, it is just as important, for your in-laws' safety, that they adopt, to at least some degree, cleaner hygiene practices. Perhaps they have had no problems heretofore but, with such unhealthy habits, it's only a matter of time. Let them know that, as they get older, their own digestive systems become less tolerant of the bacterias that will grow and thrive on the foods left out at room temperature. If not now, they may, somewhere down the road, begin to experience the same symptoms others do now. Both you and your husband, but especially the rest of his family who live closer to them, need to tell them that you are all concerned for their health. Instead of avoiding the topic (and the food) by eating elsewhere, perhaps a family conference with your hubby's sibs about the situation might be in order. Discuss ahead of time what you see as the basic problems and be prepared to offer some solid, and easily manageable solutions. (Go out and buy some storage containers or a vacuum sealer, etc.) Gather brochures and other information on the dangers of poorly prepared and/or stored foods and what to do to avoid them. Then, after the sibs' meeting, everyone should approach Mom and Dad collectively, providing a united front, and tell them you are all concerned about their food hygiene practices and the dangers it presents. The sibs should tell them how they avoid having to eat at Mom & Dad's and why. Be sure to select the most diplomatic one or two to spearhead this discussion. It won't be easy for the parents to accept this apparent criticism from their children and they will resist so be sure they know you are telling them this because you all love them and want to see them safe and healthy and able to enjoy their grandchildren for a long time to come. This, btw, is probably a step you will want to take AFTER the holiday festivities are behind you or you run the risk of eating at Denny's with your kids and looking for a motel to put up in until you can leave for home.
Good luck with this very sticky situation.