Could it be rear facing seats in the front due to airbags but forward facing ok in the back as airbags come from the side. I know in some European countries children who don't need a child seat cannot sit in the front until they're a certain age/height.
I'd email the Swedish authorities as advised and also those of any country you may need to pass through.The Scandinavians as a whole do English better than the English.
Just occurred to me if you do need a special seat maybe you could hire one.