I'm not a good musician, I just know how to read sheets and I play a little of some instruments. I have a thing or two to tell you if you want to learn accordion or trumpet.
My father is a musician, he played brass instruments, mainly the trombone, but also trumpet and euphonium. He thought it was a good idea to have me learn music so he bought an accordion (80 button, standard bass, keyboard, Somenzi accordion) from a friend and hired an accordion teacher with whom he worked with in some carnival gigs. I think I was 9yo at that time.
I went on with accordion classes for two years and then stopped. When I was 13yo I started studying trumpet in the school band and started playing. In my senior year I was the band captain. I like trumpet the most.
In Brazil there is a great book called Music Basic Principles for the Young ("Princípios Básicos da Música para Juventude") by professor Maria Luisa de Mattos Priolli. It's easy, it's simple. I don't know of any international equivalent.
You should also use a solfeggio method, like the Bona.
I only know about my type of accordion (there are a lot of accordions). I play a keyboard standard bass accordion. Things can get very complicated with bayan system and chromatic accordions.
First checks on the accordion:
I think that's a quick start on the accordion.
Brass instruments are difficult. But they're awesome.
The Arban method is great. Clarke and Saint Jacome are great books also.
Practice slowly, and calmly with a lot of focus, you can get tired easily (physically and emotionally), and usually you are not getting much from practicing this way.
It's better to invest in lots of short sessions than a couple of long ones.