It's easy to attach these numbers to just about anything. 

Use hot glue for a quick and temporary solution. It'll hold the numbers in place for the space of an afternoon party but won't hold them forever. For a more permanent solution, use E6000 glue and jewelry wire to keep the numbers in place.

For the crown, I found a pretty one on Amazon and then added a resin seashell to back the numbers and hide the battery case.

For a necklace, I used a bunch of findings and fasteners from my craft room and held it all together with wire. Be sure to keep your on/off switch accessible, and leave an opening for battery changes. This part can be fun to do together with your kids as well!