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.