Our dd, nearly 5, is on movicol and has been since she was about 2. She's had problems with constipation since she was on solids at 6mnths.
She is now down to half a sachet every other day, with a poo being produced most days! This always takes a serious amount of sitting on the toilet, cajoling and encouragement...it's tiring, especially as anything can upset this routine. We are still not back into the routine after Xmas and a tummy bug.
what helped us to get things significantly better ( because believe it or not, this is better!) was the following:
a daily probiotic, not yakult or anything like that, but proper tablet or capsule one.
daily multivitamin
daily tablespoon of linseed ( hidden in breakfast usually. Brilliant stuff! Aldi do it, in pouches, last ages, plain and with berries etc.)
Senna, perscribed by the consultant once we got down to a sachet a day. She has this but I really don't know if it works the way it should.
Its taken a while, we've been doing this for nearly a year and whilst we've gone from nearly daily soiling and wetting which made me cry most weeks with desperation, it's now only an occasional thing, although she's still wet at night and occasionally soils at night too. We keep going with the routine and duet changes, then if she get bunged up we bring out the big guns - more movicol, more henna, prune juice, special porridge with bran etc.
We've had to change her diet - no white bread, rice or pasta, fruit as much as we can, oranges, pears, apples. Egg only every few days. Lots of encouragement, games and breathing/blowing bubbles to get her to relax on the loo.
I can't see an end to it to be honest, I think the diet etc will have to stay, but that's fine, we just do it now. The movicol, we have been down to half a sachet every two or three days before, so the're is hope.
I would suggest dingbat very gradually and looking at his diet too. You are right, I thought when her bowel reduced down it would be over, but it isn't happening for us, sometimes her poos are bloody gigantic!
Good luck though, it's a nightmare, we have her ds due in a few weeks and I'm dreading more poo, and him being the same. I would have thought after 5 years I could stop carrying around wipes and spare clothes, but here we are doing it all again!