We need to see your crash log, which is created after a crash.
Open your .minecraft folder (ask us how to get there if you don't know) then open the crash-reports folder found there. Then open the most recent text file and copy its content to https://paste.ubuntu.com. Click "Paste!", then share with us the link (the URL) that it takes you to.
Yes, you will need a separate Minecraft license for each player. It will need to be installed on each user profile on the computer. Each license needs a separate e-mail and login.
Take my advice, I've set up Minecraft for families before. You will only get grief and loss of sleep if you have two copies on one computer. The kids will only want to play at the same time, so do yourself a favor and get them each a decent laptop to play on, then they can join each other's world in online mode as well. Believe me, they will only be asking for that in a month or less if you don't. Then prepare for them asking to run mods, that's another whole kettle of fish.
Rollback Post to RevisionRollBack
To tell them how to live is to prevent them living.