The nuts on kids’ bike axles have either metric or English threads, and figuring out the right size can be difficult because they are similar-sized. Visit a dealer if you cannot measure this accurately yourself. The FollowMe Tandem set comes with the 5/16 and 3/8 axle nuts included.

5/16 Inner size axle nut 6.9mm: Common
3/8 Inner size axle nut 8.5mm: The most common size
M8 x 1.25 Inner size axle nut 6.7mm: Used infrequently
M9 x 1 Inner size axle nut 8mm: For children’s bikes with a hub dynamo

Front wheels with a quick-release axle need the quick-release adapter.

Finally, there are front wheels with an M6 socket bolt, these need the insert shafts M6.