Paint Thinner?

plus you can use 70 percent rubbing alcohol if it is acrylics, or just plain distilled water works if you are using a paint brush.  The trick when brushing is not allowing your brush to be over loaded meaning too much paint, also the quality of the brush is a Major factor in my opinion.

I only use real hair brushes for painting figures and they have to be soft (I like mine soft others differ)   I would suggest looking at the type of paint see what the paint to thinner ratio is for an airbrush and cut the thinner down by 1 or 2, as you do not want the paint to be runny but kinda muddy (so one else will be able to explain this better i am sure)

Hope this helps.  Oh and if you guys are going to be doing more and more models think about a cheap airbrush/compressor (don't waste money on the cans)  you will find that you will love using it and so will your daughter, as mine 10 yoa, still comes running when I fire mine up to see what I am painting.

Depends on the paint.  Some brands are okay to brush paint as they come, others do require thinning.

 There is an ongoing debate as to what kinds of thinner to use with various paints (what type are you using?). Some of us say any kind of generic paint thinner is okay, others of us recommend using thinner of the same brand as the paint.  There, this post will probably bring up that discussion again :-)