NHS Choices is a great place to get information about common childhood illnesses.
For example, their section on common infectious diseases, which also has some slides showing common baby rashes.
There’s also their excellent visual guide to common childhood illnesses.
For guidance on how long to keep children off school for various illnesses, click here.
Here’s local GP, Simon Bowers, in 3 useful short videos about how to manage common childhood illnesses. They cover coughs and colds in children, diarrhoea and vomiting, and high temperatures and fever: