First mistake was in asking the kids! Now you'll have to ignore their answer! I agree that 5th graders will have trouble with this, as it's a little juvenile for them and they are also sick to death of going there from years before. Do these kids all know each other, or do you tutor them in separate sessions so that the only thing they have in common is that they are your students?
You didn't mention, but is there any expectation on you as a "teacher" for providing healthy foods or a healthy environment?
It's going to be challenging to have activities that everyone can participate in, with such an age range. You might consider something like a local museum (science museum, children's museum) or zoo - have groups and chaperones, let people roam as they wish and then all meet together for a picnic. It could be a BYO brown bag lunch, or you could provide the basics and let people bring their own additional items. For example, in our area there is an environmental science museum with an outdoor wildlife area and playground, plus a picnic area with tables, and a snack bar. There's also an indoor lunch area which is good if the weather is poor.
Another option might be mini-golf - break the kids up by age, and limit the little kids to 5 or 6 strokes before they pick up the ball and move on. Bowling is also possible - a lot of places in New England have candlepin bowling (smaller ball and pins) - don't know if you have it in your area. Standard bowling alleys sometimes have somewhat smaller or lightweight bowling balls, and put guards in the gutters so the little kids can't bowl gutter balls all day and be frustrated! Adults can supervise each group, keep score (if you bother) and so on. For mini golf you'll need a rain date of course.
Another alternative is a park and picnic - you might be surprised at how kids of all ages enjoy old fashioned games like 3-legged races, sack races, potato races, and so on. I got a book from the library to remind myself of the suggestions and rules, and we had great parties over the years because today's kids had never played those games. If you have time to prepare something in someone's yard, you can do treasure hunts - I used to buy those plastic Easter eggs and use them for prizes. Older kids can help the younger ones, or you can separate eggs by color and give groups a few colors that are "theirs" so that everyone gets something. I have separated primary colors for younger kids and pastels for older kids. What's inside can include little candies, coupons, or silly toys from places like Oriental Trading. (OT sells stuff in bulk, but you could save some of it for next year! Also, around here, people give out toys at Halloween because of the obesity issue and a number of neighborhood kids with diabetes. So it might be an investment that would pay off. You can also deduct this from your taxes as a business expense - that is, for anything you give to your students 'tho not for Halloween!).
Good luck!
