Went thru this as well, and all our kids have had varying degrees of impaction.
Enemas are more hurtful than helpful to small children! Add to that that enemas can be painful, SCARY, and stressful.
The doctor's at Children's Hospital here had us use Glycerin Suppositories (just plain vasoline bullets). Less stressful to the child AND the parents. The suppositories allow the movement come out easier, with less friction-pain, and lubricate the bowel, making it a quicker passage. The body doesn't become dependent on laxatives, and learns to "self-regulate".
The two largest factors that affect bowel movements in all of us is the amount of physical exercise we get, and most of us don't drink enough Plain WATER. (The sugars in Juice and such can actually compound the problem.)
We had a wonderful Family Doctor, who believed in using "home remedies" before resorting to prescriptions. He had us do something that is SO simple, and once you stop and think about it, actually makes sense. Allow your kids, especially toddlers who are still "mouthing" everything.....play in a Warm Bubble Bath until they are tired of it, and want to get out. SMALL amounts of soap (Ivory, baby shampoo, glycerin or castile, even plain dish-soap, - NOT soaps with fragrance, additives, etc) like chewing on a washcloth while bathing, Soap-bubble beards, and so forth. It isn't harsh on the systems, doesn't kill off the necessary organisms, lubricates the intestinal tract, also helps keep more water in the bowel tract, softening the movements. The warm water relaxes the muscles of the abdomen, and de-stresses the child. After about a week of nightly bubble baths, our kids would get out of the tub to go poop, and then climb back in to finish playing.
We also had to Teach them to go regularly. Children should have at least One movement Every Day. (Adults can go less frequently, but not kids) Find a time of day that is convenient to THEIR schedule: after breakfast and before school, after baths at night, whatever. And just as we have to remind them to go brush their teeth, start reminding them to go Poop. Once their body gets into the habit of going at regular times, the frequency of them being constipated will decrease, or go away all together.
There IS a laxative that is safe to give 6-week old newborns even, and has no dependency effects. Polyethy/Glycol. It required a prescription, comes in a powder than mixes with milk, juice or Kool-aid, is tasteless, and works within 12 hours. It was the ONLY laxative our Family Doctor would allow our kids under 4 to use. Fletcher's Castoria was his choice for the kids 4 and up.