Get some foam shapes from the pound shop - make sure there are 2 of each shape - ie - 2 red hands / 2 blue stars etc - 1 pair per child. Stick the same sticker on each pair if you want to toy story theme it. Put one of each pair on the floor around the room and give each child the other one. They have to find the other one, bring you their pair and they get a small prize. If you like you could then put out some more stickers and get them to decorate their pair to take home.
Actually, the pound shop if brill for craft stuff - find something - anything! - for them to decorate with stickers. At a recent party, dd was given a small polystyrene ball (pack of 12 = Ā£1) to put foam shape stickers onto.
Also, Swim Swim Fish worked well at our party - the children go to one side of the room and you tell them that you're going to say 'Swim ,swim...' and then an animal If it's an animal that swims, they get to run to the other side of the room (you may have to do this too to encourage them). If it doesn't swim, they have to stay where they are.
Also, sweetie hunt around the room.
Sing some nursery rhymes for them to join in, and - always popular - ask them if anyone wants to come out and sing their favourite song. If you have a microphone, even better. If not, it doesn't matter - or you could make one with a kitchen roll tube and ball or get an inflateable.
Have lots of balloons. As they arrive, put on music and let them just play for a bit until you're ready to start. You can also do balloon dancing / volleyball (ie - who can keep their balloon in the air the longest).
Who can moo the loudest? / sit the stillest / stretch the tallest / make the smiliest face / scariest face / balance on 1 leg etc etc
Glowsticks / glow bracelets make fab small prizes - get them off ebay.
Hope that helps!