I don't have guppies and I've never had ich in any of my tanks while having fry in them. But I would say you should treat the tank and the fish... ich could kill them! So just raise the temperature slowly so the fish can get use to it, treat the water, and leave it at a high temperature for at least 5 or 6 days.
I hope this is not bad advice (with the fry and all) but I'm sure the forum members will correct me if I'm wrong.
Good luck with the ich!
You can check this out for more detail regarding ich etc.
http://www.fishlore.com/Disease.htm