Quick reference
NHS England - National Primary Care Clinical Pathway for Constipation in Children
Seems to be an excellent resource, made in collaboration with a number of organisations and charities
Page 3 - Flowchart
Page 4 - Red flags
Page 7 - Disimpaction guide
Page 8 - Maintenance guide
Patient information
Eric - How to use macrogol laxatives a good guide on how to and importance of making up macrogols correctly, and how to disguise their taste / add them to other parts of the diet
ERIC - Baby and toddler poo - what to expect
ERIC - Constipation in Children
ERIC also has lots more resources on bowels and bladder.