I took her to the doctor after nearly a year of complaining about tummy ache, but it suddently got every day rather than just when she didn't want to do something. It seemed to start after she'd had a urine infection, but I don't know if that was connected.
She was 4.6 when we took her and the doctor did bloods, and also referred her to a paed, who looked at her, felt her tummy and sent her for a scan which confirmed faecal impaction.
I wouldn't have guessed she had constipation as she seemed to go fairly often, not every day, but obviously I wasn't monitoring the amount etc like you can do with nappies. 
What I had noticed was that she would sometimes say she needed to go, then get the the toilet and say she didn't need to after all.
She came totally off the movocol last summer and she's 8 now. (7 then) I think she had about 18 months on 2 sachets a day, after she'd been cleared out. Then we reduced it down to 1 sachet a day. Then 1 every other day. If she didn't go for a couple of days I'd give her an extra sachet for a couple of days and then reduce down again when she was back to normal.
I've just got the info out of the last packet again, and, asuming you have the same packet as me at the bottom it gives a daily dose regimen for faecal impaction. We got to day 3 when it came out-we did it over a long weekend so she didn't have to have it at school. We were told give her large doses until it's coming out like water to make sure it's clear. If you're giving a large amount then I'd keep her off school or do it over half term as it does pour out without much warning.
She did still occasionally get tummy ache even after clearing, but the doctor thinks she gets abdominal migraines. Certainly tummy ache comes on with stress or excitement, but we have learnt to recognise when it's constipation not other causes. But it took us some time.