As we are still unpacking after moving houses, I couldn't put my hands on any paint brushes, but then remembered how much children enjoy using cotton-tips to paint with. They are easy for them to handle and they create fine, delicate lines.
We used coloured dye for this activity, putting small amounts of dye (that I had mixed up from eticol dye powder) in glass jars. Food colouring drops mixed with a small amount of water would be just as effective.
For something different, Finn decided he wanted to use a sponge roller to dip in the dyes. It was much more difficult to handle, and the lines a lot chunkier- but it created an enjoyable painting experience for him.