I enjoyed teaching a session at @UOSEnglish yesterday on query letters and 1st chapters. There are a lot of query letter rules if you google, but you really don't need to overthink it. Here is a very basic template for commercial children/YA submissions to start you off

The pitch part definitely gives writers the most stress, so try to think about it as a jacket blurb. What does your character want? (goals) What is stopping them getting what they want? (antagonist or problem) What will happen if they don’t get what they want? (stakes)

A very simple example:
“[Some sort of descriptor plus age] character name wants SOMETHING, but SOMETHING is stopping them getting it. If they don’t get it, SOMETHING BAD will happen. [optional X meets Y]”