Hi A.,
I have a 6 year old son who walks on his toes, and it has been a frustrating process getting answers about the cause & best treatment for him. The place to start is a pediatric orthopedist (I don't know if you are in the Twin Cities area, but we saw Dr. Teresa Capello at Children's St. Paul & were extremely happy with her). Our (former) pediatrician didn't even look at my son's feet when I brought it up at a well-child check, but I insisted on getting a referral to an ortho & am very glad I did!
The things to look at are: has your child always walked on his or her toes? is it the same on both feet? (my son actually told me, "I can't put my heel down on this foot, it hurts too much.") if you ask your child to put his or her heels down, can he or she do it and walk with a normal gait? If the answers to those questions are yes, then it is most likely "idiopathic toe walking" (meaning no one knows why they do it), and there's a decent chance no intervention will be needed. In my son's case, he started walking normally & then started toe walking at age 2 or 3, and as I said, one foot is quite a bit tighter than the other, so we have seen a pediatric neurologist & ruled out lots of things. A developmental pediatrician is another option. Unfortunately, there isn't necessarily a diagnosis in these cases, but the best we can do with my son is call it mild cerebral palsy & go on from there.
If they decide that it is NOT idiopathic toe-walking, the best treatment is serial casting, where they position the child's feet very carefully, put on walking casts, and repeat weekly or every two weeks until they're happy with the position of the feet, and then custom braces at night until the child is finished growing (keeps those muscles that we worked so hard to loosen up from tightening back up at night, as they grow). The serial casting usually takes 4-8 weeks. They do not like to do "heel cord lengthening" surgery any more, as it is a one-shot deal, and does not address the tight muscles, just the tendons (or ligaments?).
As you can see from the length of my response, I have some experience with this issue! Please feel free to send me a message if you have more questions.
Good luck!
A.