Having worked with very large groups of children of mixed ages, this is my suggestion:
First of all, think of this as "just for one day". Just for one day, it IS okay to have movies on all day, most especially if you are going to be busy with other details and the other adults don't feel so compelled to mind their kids. Have each family bring a 'favorite movie' for their kids. Send very clear guidelines via email so there's no confusion (no scary movies, harsh language, etc--whatever your families preferences are). Having a stack of favorite movies is a last resort, but useful.
Invite each family to have their child bring one or two favorite games. This works well in lieu of toys, as games are meant to be shared and Christmas toys are often very special to the child.
Make a rotation chart and explain it beforehand. Ask parents to sign up to be the Adult Eyes and Ears, (each family can have one parent do an hour-hour and a half shift).
If it were me, I'd be fairly honest with people about what is needed to pull this off. Let them know that it will be fun for us all to visit, and the children really do need a dedicated set of adults to watch them. (Maybe signing people up in pairs to do this so they don't feel alone?) I'd also ask people to bring a ready-to-serve dish and do some of this potluck style. This could save you a lot of work and I think it makes people more thoughtful about what's going on instead of showing up someplace to say "let's eat".
One more thing: I directed childcare for religious services recently and we had just this, mixed ages and a high number of children. We were staffed at a 1 adult:4-5 children ratio and this was necessary for the relative success of the program. Know that you do not have to be superwoman and that it is Perfectly Reasonable to ask people to step up. Discuss this with your husband and have*HIM* send the email out to his family.Coming from him, a family member, will protect you and your relationships with his family. The bare bones of the matter is that childcare for so many children IS work and it all goes so much better if we share it.
Happy holidays!