What are the most important user interface design principles for designing for children?

The best way to know your audience is by deeply understanding their developmental stage. Age is a label we use to categorise users but each and everyone of us reaches critical developmental milestones at different times. Effective UI must consider both physical and cognitive development. Remember just how little those fingers can be and that a single mechanic - tap or swipe being best - is always the way to go for younger children. All the insight you need about development and the milestones children reach is in the public domain. Read it, know and then use it to understand the "Why" in the observations you make of the "What".

Colour is of course very important but what people often miss is that it isn't actually about bright colours. It's all about the difference in luminance between the colours. As a child's visual acuity develops so they can properly distinguish kaleidoscope but at the younger end ensuring that there is clear contrast in luminnce and not just colour will pay massive dividends and they will attend more to the visuals you've created for them. You can test your creative by looking at it in greyscale - if it all washes into a very similar range of greys you need to tweak the contrasts.

750 characters is a hard ask to do this fundamental subject justice. So I'll say this...regulation globally is in flux and big tech seems to be very slow in ensuring the safeguarding of children. Simply identify the regulation in your territory and then go beyond it...make it even more robust that law makers require and let parents know. And if you possibly can just DON'T COLLECT PERSONAL DATA. But if you have to ensure the highest global standards of privacy. The UK Online Harms Bill is a good place to start.

Completely agree and the intention and actions to go beyond legal requirement has to be the mindset for this. Ensuring the safety of our children and therefore the future for all has to be priority 1.

When testing with young children I find it's more important to watch what a child is doing than to listen to what they're saying. In particular, you need to pay attention to a child's facial expressions, what they're doing with their hands, and what their body language is telling you. You will discover great insights simply by watching.

If a child taps somewhere on a screen, and there is no response, that's a missed opportunity for engagement. Sometimes graphics look like tappable buttons. If so, code a response into that item, however minimal, but preferably with a meaningful response.