Constipation/impaction/soiling/Movicol - feeling at my wit's end

[TheLightPassenger]

I have an 8 year old with chronic constipation - fussy eater, hates veg, sometimes has to be nagged to drink enough, which really doesn't help matters. After over 18 months on Movicol, and episodes of being close to sorted, it all goes wrong again . I'm particularly worries as DS is going on a residential next week, and we are going through a bad patch. School are aware and v supportive but am dreading the reaction from his peers. DS now showers himself after an accident, but has to be prodded to make sure the bath etc is properly clean with no traces of poo . I just don't know what to do. We are under the continence service, but they are proving hard to get hold of, I have left a couple of messages and waiting for them to get back to me. DS is currently on 2 movicols daily (upped from 1). I have no idea whether he is on the right dose, too little or too much/

keeping - have also heard that coconut water can do wonders as well (you can now get it in bigger supermarkets as well as health food shops) but not tried it.
Otherwise, try and up fiber content gradually - there are lots of non fruit and veg foods that are reasonably high fibre too - weetabix bars, digestive biscuits (including chocolate!) all bran bars, tinned spaghetti (spaghetti seems to have a lot more fibre than rice in btw.) If your kid doesn't like wholemeal, the bigger supermarkets do pasta and bread that looks non-wholemeal but isn't iyswim.

keeping - the really big doses of movicol are sometimes needed for impaction, (TMI description coming, sorry) to get things going when a lot of old solid poo has built up that is hard to pass, and softer stuff is leaking out around it.

use movicol to keep LS going, problem is, she soils constantly, as poo very sticky and loose, and i don't know how i could find out if she is compacted higher up in her gut? so worried about sending back to school as she is nine and kids are bulling her about smell