Hi and welcome!
There will be children's books explains diabetes, for different ages, available. Although I've never looked for one myself.
I think the most important think when explaining about diabetes to anyone of any age, is to handle it without drama. Children are amazingly accepting of things, and just slot them into their worldview like they are just adding another brick to a wall. If everyone just quietly and casually acts like diabetes is normal, and those are Mummy's special medicine sweets/biscuits (maybe buy distinctive 'medicine' brands of hypo treatment sweets).
I've no personal experience of this - type 2, no kids - but I babysat for years for a woman who had leukaemia. Her tactic with the kids was brilliant. She just handled everything (hospital visits, drugs, nurse visits, hair loss, nausea, bathroom trips, illness, everything) as if it was utterly normal, and maybe a teensy bit special. And she made sure that the kids never had a second's worry that school, meals, tv, toys, etc. would carry on as normal, even if mummy was on a different routine. The kids had reassuring stability. It was a very impressive achievement, and really paid off.