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
Common, for shafts with an outer diameter of 7.938mm
3/8
The most common size, for axles with an outer diameter of 9.525mm
M8
Used infrequently, for shafts with an outer diameter of 8.00 mm
M9
For children’s bikes with a hub dynamo with an outer diameter of 9.00mm

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.