I introduced oil pastels to Missy (aged 3) and she LOVED it. A bit messing so make sure you put a drop sheet down, have an apron on or smock or old clothes and wipes on hand.
Oil pastels are a great way to introduce blending and smudging. I tried this with my three year old and she had a ball
A great activity to do with older primary school age students is to introduce contour and tone through simple exercises using a circle shape or fruit shapes such as oranges and pears.