Kids who aren't old enough to sit in chairs

I would keep them in a high chair until maybe age 3. If the child is big, I would ask the parents. Don't they have those short chairs that the car seats sit on for infants? Sitting one of those on a chair isn't very stable.

If the parents RSVP yes for any child under 3, I would contact them directly to find out what they need-- booster seat plus chair, high chair, or a chair for them to place their own booster/high chair attachment on. Most rental companies also rent high chairs and boosters, though unfortunately these tend to be more expensive than reg chairs (I've been seeing $9 for high chairs vs $3.50 for wooden folding chairs), so if parents can bring their own boosters/attachments, that will save you some money and some trouble.

I agree with Barbara

Ask the parents of the children. My daughter is 3 and wanted to just sit in my lap from about 6 months until around her 3rd birthday. Now she is about half and half with her own chair or my lap, depending on her mood. Ask the parents, they know their kiddos the best.

If you get high chairs, a high chair can be turned upside down, and a car seat sat on top of it, for a child too young to sit in a high chair directly.

I agree about contacting the parents directly. My sons first wedding he was 3 months. And they accomdated us by calling and asking us what we would like for them to do. The next... He was already 1 1/2 and they know him pretty well, so they just had a high chair next to us (we didnt end up taking him, mommy and daddy wanted to enjoy a much needed night out)

IF the families are local see if they have their own portable high chairs that strap to a chair with them. Many families do have these, and booster seats too.

Also talk with your venue and see what they have. I worked an event last night where I did actually see some that were used for a few toddlers there. So this could help in particular if you have guests from OOT and might not bring it with them on a flight.

Consider small baskets of table top toys near them to keep them focused on playing while waiting through the meal.