1. No, you just need to setup your kids as your "Friends" by sending our invitation and response once. It is permanent "Firends". Once set, they are always in your "Friends" list and you can check their location anytime (as long as they have cell signal I believe).
You can also setup with your friends as "Temporary". It has a time limit, for example, for 1 day or 2.
2. see 1.
3. No. Keep in mind it is a very basic app built in OS.