Much more of a celebration than most of us would have here in the U.S. But as for decorations and food, go with a nice spring motif. Don't get too carried away, you're in a chateau, for heaven's sake, let it be the star. Put out some flowers that are in season at the time, but let the girls and boys in their finery be the real centerpiece.
Make your deserts fruit oriented, especially seasonal berries. I would make up some kabobs in advance -- this is a crowd pleaser since you can make some with just vegetables, some with lamb, some with chicken, etc. It makes the grade as far as the different cultures go. It is also the kind of finger food that children like. As a matter of fact, since your cooking is limited, I'd go with kabobs, period. Put berries and chunks of other kinds of fruit on skewers for the desert, and have some whipped cream or topping (I'd buy the frozen kind and just thaw it for my crowd, they prefer it to the real thing) to dip them in. Then make up meat and veggie kabobs to grill. Then I'd have some kind of starch-based salad (potato, pasta, or rice salad); once again, something easy to make up in advance and easy to accomodate many different cultures and religions. My favorite dressing for pasta or potato salad is half mayonaise, half yogurt, with lots of chopped herbs added, and maybe something tart like pickles or green peppercorns. For rice salad I find converted rice easiest for the purpose, dressed with an Italian-style dressing. Both get whatever vegetables are pretty when I go to the grocers.
All of this stuff is easy to make in advance, and very kid-freindly.
Funny, I don't really remember anything we ate at my first communion. I do remember one of my little sister's, but not the other two.