That's a good idea in my book, my adult cat was recently runover, it's heartbreaking.
Some people don't use collars at all here, sometimes it can cause balding around the neck, but that's your choice. My two wear collars. As much as you intend not to let tham out, it's still best to get them microchipped *just in case*. I would treat them with frontline again soon, and then leave it for the winter. Start again around March/April. You can get household sprays and stuff quite cheap if you want to do your home over the flea-season too.
If you have a bagless vacuum with a filter up the middle, wrap the flea collar around it backwards, so's that the *fluffy* bit is facing outwards.