Super quick and easy to set up, minimal mess and cute results.  Bend a cardboard tube, toilet roll for example, and fix the top with some tape to keep its shape.  If you have some heart shape cookie cutters, they will work too.  We also made 2 potato stamps.  Some coloured paper, paint and let them at it.

Paint

Heart shaped toilet roll, held in shape with tape.

potato stamp

Mr 2 stamping with potato

Ms 4 stamping with toilet roll

Different colours

Patterns

Sone of the finished cards

Finished products,

Finished off with a sharpie

The potato stamps worked out much better, as neither of the younger 2 had the dexterity required to get the roll to print properly.  You could finish them off and write messages on with a sharpie as I did in the last image, or you could just use them as decorations rather than cards.