Actually many children I know have a difficult time writing many of the numbers because they don’t have many opportunities to write them like they do with letters.
I think that especially when kiddos turn five–and when they’re ready to head to Kindergarten–it’s important that they know how to write ‘their’ number.
Lots of practice, many varied techniques, and making it all silly–and fun!–so they don’t even realize they’re learning has been a focus for us since the kids were little.