Hi
Charlene1234, welcome to FT! And sorry for the incoming wall of text!
I don't have answers to all of your questions, but I do know a bit about flying on and earning miles on United (UA) and can provide you with some basic information.
Each individual who flies earns the miles, so make sure that you and your children each have their own UA MileagePlus account. If they don't all have mileage accounts, you can create one and then add their account number to their ticket either online, at the airport, or on the phone.
UA, and most other airlines, no longer base earning miles on miles flown. For earning miles, UA credits 5 miles per dollar spent on the airfare (for basic members with no UA status), not including government taxes and fees.
For redeeming miles, UA offers two different types of award ticket: Saver and Standard. For domestic flights:
Saver tickets are 12500 miles one way for an economy seat and 25000 for a first class seat.
Standard tickets are 25000 miles one way for an economy seat and 50000 miles for a first class seat.
While you can generally find standard award tickets, saver awards can be more difficult to find depending on date of travel and route flown. Inventory is generally limited and fairly dynamic. If UA thinks they can sell the seats to paying customers, then they are less likely to have a large amount or any saver awards available.
There is a wealth of information on FlyerTalk. There is a dedicated United Airlines forum which can be found here:
United Airlines | MileagePlus. However, this is probably the best page to start within that forum:
United MileagePlus Master Wiki.
You'll see a lot of abbreviations, acronyms, and terms thrown around here that probably won't make a ton of sense. Here is a link to the glossary, where you can look up these terms:
FlyerTalk Forums | Glossary
As for the multiple posts. I just picked this one as it seemed to be the best one to respond to. It's not possible for users to delete posts, but moderators can. I've reported the two other posts as duplicates. In the future, if you need to alert a moderator or report a post, you can click on the

image on the left side of the screen.