These are self-explanatory, and you might still remember doing this in elementary school, so, I'll just write some tips.
- use embroidery hoop to support the fabric when sewing on buttons.
- use blunt end needle, like the ones that are used by knitters.
- start with the buttons that have 2 holes.
Draw the diagram for the 4-hole buttons combinations and help the child figure out the directions.
make buttons from heavy cardstock (cut the circles and use a hole-punch to make the holes) and use a sheet of plastic canvas for the fabric. Use thin shoe laces or a plastic needle with yarn to sew.