I am going to be seaming an aran sweater I made for my 3 year old and I am not sure what order to go in I know the shoulders come first but I am not sure of the arms and body .Do I do the body first and then the arm seam so that I am fitting the arm into the arm hole?

I would do the armhole first, while you can still lay the sweater out flat. Then sew the sleeve and the sides.

Start the sleeve seam from the cuff edge and the side seam from the bottom and have them meet at the armpit. If there’s any discrepancy in length, you can fudge it there, where it will be less conspicuous.