Hi D. Welcome!
Well, let's start with the critique part so you have links to look at.
You've got enough posts to create your own post in the
Children's SYW subform of
Share Your Work (password=vista) where other MG writers (us) will take a look and give you feedback. Within Share Your Work also lies
Query Letter Hell SYW, where you can get feedback from some very knowledgeable people about your current query letter. If you're looking for a Beta Read of a greater portion of your work, you can also post a request in
Beta Readers, Mentors, and Writing Buddies.
What most people here will advise is that before you start your own post in SYW, you look at some of the other posts there and offer feedback. This serves several purposes: 1) By not only critiquing other people's work but also watching it get critiqued by others, you learn to identify what might be basic issues in your own work. If you can fix these issues on your own before you post in SYW, you'll get more meaningful crit on deeper issues rather than surfaces ones you could have already fixed. 2) If you show a willingness to critique other people's work, thus giving to the community before you receive, you're likely to get more and more experienced critters on your thread (we like to help people who help other people). 3) Watching and participating in critique in an objective way, first, lets you see the process of it so that when it's your turn to get critiqued, it won't sting as much and you'll have a feel for the etiquette in receiving crit and thanking critters.
You might also get some comments here about waiting to query further until you've completed your third edit and feel your work is the best it possibly can be. Many people feel it's wasted effort to send out work you know needs more editing because you'll potentially burn the bridge with that particular agent for that particular work if it's not quite ready to query.
As to what the rejection comments mean... it's difficult to tell from such short snippets. They might be anything from kindly worded form rejections, to agents who genuinely liked your work but didn't love it quite enough to take it on, to agents who loved it but don't quite have the connections to sell your particular type of story but know surely someone else in the industry will. Form rejections are general and vague. Personalized rejections are a little more specific and sometimes mention details from the manuscript. If it seems specific enough to have been personalized, then I'd take them at face value—i.e., you probably just haven't found the right agent for it yet.