Growing up in Minnesota we always said things like "Oh, for sad!" or "Oh, for cute!" which mean "oh, that's really sad/cute" or "oh, that's too sad/cute". I wonder if that usage has its roots in the Scandinavian heritage we have in Minnesota? Either way, the usage of 'för' in this example made me think of home and smile.
Um, I just said that you can tell here because we don't really use små as a nominalized adjective. If it had said Byxorna är för små barn, you could have been sure it would have been 'The pants are for small children'. So as a rule of thumb, when it's för + just an adjective, you can assume it's too, but when you have för + adjective + noun, it'll be för. (This is a bit simplified and won't cover every case, but it's a start.)