It's likely a urinary tract infection, OR you need to add another litter box. The general "rule" is you need at least the number of litter boxes as you do cats. As many get older they get much pickier about their box. If there is a lid, take it off. Add the second box. Scoop both boxes twice a day, each and every day. Does the dog prevent the kitten from getting to the box? Does your toddler do anything like that also? This includes "watching" the kitten use the box, and chasing. If you do all of these things and the behavior continues, then you need a vet visit to rule out a urinary tract infection. It's very common in male cats, especially those fed an all dry diet.