Would the problem with acrylic paint not be that when they dry, and they dry very quickly, they are no longer water soluble, so they are really only usable while they are wet and liquid? It's the consistency of oil paint, but, as you say, water soluble, but with very fast drying qualities. You can buy reasonably priced student water colour tubes and, as HildaW says, squeeze them into an old ice cube tray and let them dry. They'll last for ages and the colour will be better than any child's paint box. The Works sell good, reasonably priced water colour tubes and sketching boxes (little square pans in a box).