Cat,
how much space do you have for activities?
If they're all at the dun-wan-to-share stage (like Jesse) then steer clear of big toys like cars and such...they'll fight.
Don't think it'll be such a "long" party, right? About 2 hrs? I find children are not very particular about toys (new or old, fun or not, etc) they just like to try new things...so for other children, Malcolm's current toys should suffice. If you want to keep them (which might be a good idea if M doesn't want to share) then play a DVD (Madagascar/Cars etc) or some music, maybe playdough/stickers/crayon and bubbles...children are really easy to please. Probably good to stick to activities that don't require too much parental supervision, that way the adults who visit can also relax and chit-chat.
If you have space, like a back yard, can fill up an inflatable pool with balls...or very shallow bit of water (<10cm)...
As for food, just cook a whole lot of porridge...(so far the parties we've attended, there's usually porridge that the children will eat and then they'll nibble at whatever the adults eat.) if you're talking 2 year old kids lah. If they are older, can consider foods like fish fingers, nuggets, macaroni with tomatoe sauce, sandwiches...if they are older, they could probably eat from the buffet as long as there are a few non-spicy dishes.
Have fun with the planning!