but in all honesty here i would like to answer the question.
i would ask child B to grab me some of the "free" materials.
I would ask child C to grab his tools or whatever he needed
I would ask child A to run home and grab his music book.
Then,
child B cant afford anything - there is no loss to the materials he gathers other than time
child C makes it free of cost, there is no loss other than time
and child A will play the flute or train the other children on playing, there is no loss other than time.