Sounds like you are getting on the right track.
It is important not to blame yourself for what has happened, it isn't your fault and as someone who is new to this fishkeeping lark, you didn't know what to do and had been given poor advice by the shop workers. Don't beat yourself up.
Fishkeeping is a mix of science and patience (alot of patience)....and a little frustration thrown in here and there.
It isn't an easy hobby. It can be expensive both financially and emotionally.
You're in the right place here to gain advice and build experience......you'll get there and despite everything, I am sure that you and your family will begin to enjoy keeping fish.
Keep us updated on your progress, if stuck just ask....we are all here to help you.
Fishkeeping can be tough when first starting out....but once past those potholes and curveballs, it is an extremely rewarding hobby