Evening ladies

What sort of matress have you got for your trundle bed if you have one. DS will be 4 at the end of September. I am looking to get a matress for his trundle bed for when his little friends come for sleep overs. I am thinking as they are still so wee to just get a foam matress - won't be used that often and they are all still light. When he is about 6 or so I can then buy a "real" matress. We used the blow up sleepover beds with built in sleeping bags during the summer but now that its winter and so much colder think that it will be better using a trundle.

What do you think?