I have occasionally thought of doing the same thing for just the same reasons. I have just Googled the following: advice on writing books for children
, and a number of helpful sites came up - some are more commercially minded, others just offer plenty of good practical advice.
You might try some of those first, and then do the same search limited to UK sites only.
If any parent on the forum has had a children's book published, or knows someone who has, I would also be very interested to hear about it!