Webster's says:
Solipsism
n : the philosophical theory that the self is all that you know to exist
In Piagetian theory there is a notion called "object permanence," a conceptual understanding realized about or before the age of two, whereby the child understands that things continue to exist even if they are not presently stimulating the senses (e.g., out of sight).
Over the years, rather than rehash this conundrum, I have learned to remain silent and smile in the presence of philosophers, and reflect on the many insights that children have that some grown-ups seem to avoid or abuse for their own purposes.