V.V.
Mirilax is a laxative that works by pulling more water into the bowel, making stools softer and stimulating the intestinal tract to produce a bowel movement.
It is safe for young children; in my ICU we've used on children as young as 10 months. It's also safe for long term use.
Benefiber may be another option for you, if you want to stay away from laxatives.
Increasing fruits like peaches, plums, grapes, and prunes will help. So will plenty of exercise - lots of running around.
You might try a little rectal stimulation, too, with a well-lubricated rectal thermometer - sometimes that will stimulate the urge for a bowel movement. We do that on my ICU a lot as well.