I'm looking for a good back carrier (or other solution!) for an older child. DS is 5 but is small for his age and only 16kg. We like to go on walks as a family but he inevitably ends up on DH's shoulders because he is soooo painfully slow at walking. I have a rucksack for DS2 13.5kg) so if we invested in something for older kids it could always be handed down. He wouldn't be in it for a full walk, just occasionally when we needed to move a bit faster. Is there anything out there that is suitable?