Rather than using your old toothbrushes for yucky cleaning jobs why not give them a fun new creative job. Toothbrush painting is as easy as painting with a paint brush. Clean up your old toothbrushes with disinfectant or Milton and instead of using a paintbrush to paint, use a toothbrush : ) They will give a completely different effect to your child's painting and you're recycling at the same time. You can use toothbushes with acrylic, water based,
Cel Mix paint (from an art supplier) or any paint you like really.