Lets take the issues one at a time ...
First off, that is a brutal ticket. $465, ouch. I sympathize.
This is a great example of how officers can get folks to say things that REALLY damage your chances at trial. I'm not sure exactly what you said to the officer, but depending on what you said and what the officer wrote down, this could be tough. It sounds like you admitted to speeding (38/39 in a 35). You may have also admitted to not seeing the kid (if there was one). Finally, it sounds like you admitted to being on the phone while driving. Although the office didn't cite you for that, the judge isn't going to like it because it can show you weren't paying attention. The best policy is always NOT to answer questions officers ask at the door (especially questions like "do you know why I pulled you over.)
But a lesson in what you should have done doesn't help you. Sorry.
It sounds like you were in an area that has been posted with different speed limits "when children are present." Whether children were, in fact, present will be your word against the officer's. You need to get a copy of the back of the ticket (you can request "discovery" from the court) to see what the officer wrote about the kid. If it is a vague description, you might want to ask for the officer to come to trial and ask questions that show his lack of memory (what was the kid wearing, how old, how many kids, can you really remember that far back, etc.)
The back of the ticket will also show what the officer wrote down about your statements. Officers see hundreds of people each day and write dozens of tickets every month, so if they didn't write it down on the back of the ticket, they aren't going to remember it.
I don't think your "flow of traffic" argument is going to help. I'm also not sure how helpful testimony from your boyfriend or from you about the engine light is going to be.
Here is what I would do:
1. Consult a Washington traffic attorney. We know lots of ways to get tickets dismissed based on procedural and legal deficiencies before you ever get to trial.
2. Contest the ticket. Anything other than contesting the ticket (paying it, requesting a mitigation hearing) will result in the ticket going on your record, even if you pay a reduced fine.
3. Consider a deferred finding. Every seven years you can "defer" a ticket. Essentially, the judge says, "pay a fine and don't get any tickets for a year and this one will drop off your record." If you haven't deferred a ticket in the recent past, it can be a good deal but remember, if you get another ticket before the deferral term is over, BOTH the new and old tickets end up on your record.
Even though some time has passed since you posted your question and you got your ticket, you can still probably get a contested hearing. Do that right away.
Good luck. Keep me posted on how it turns out.