I understand your concern about your child's bowel habit.
is a poorly absorbed drug through GI tract and is excreted unchanged via feces. White color of stools is probably due to excretion of drug and since your child is eating and feeling fine, it should not be a cause for concern. You can repeat one more dose after 2-3 weeks for complete response.
Don't worry dear, worms are dead and your child is fine. Take care.