Mixing Primary Paint Colours to get Secondary Colours:
There are 3 primary colours - red, blue and yellow.
No combination of other shades can create primary colours, however all primaries can be mixed in various combinations to make more colours (hues).
Kids at an early age are usually only interested in mixing primary colours to create secondary ones, and mixing secondary colours to create tertiary.
Basic Primary Colour Combinations:
Red + Blue = Violet (Purple)
Blue + Yellow = Green
Yellow + Red = Orange
Have at the look of an example colour wheel above: