We have been through this, only just getting through it now. We used osmolax for a year, swapped to lactulose in the end as an xray showed her tummy was still full of poo. We had to completely overhaul dd's diet. No bananas, sultanas or processed food like breads, biscuits etc. we encouraged lots of other fruits, baked beans, chia seeds on everything and lots of water.
We had issues at kinder as the carers were very harsh and scared dd which is why she held on the had accidents which then caused the build up. We have had a few issues with holding on and not asking to go to school but it's much better now. it's a combination of things on her mind and forgetting to go that's the biggest issue now.