There is not a lot of good data about the onset of T1 diabetes in young children. But there are a lot of anecdotes I've heard over my 35 years with diabetes.
It seems likely given the onset of T1 in my case, and given anecdotes other here have posted, that insulin production may be "barely able to keep up" sometimes and other times "not able to keep up" for a good while (months... maybe even years) in some of our histories. While in this state, bg may go high after a meal (above 180), resulting in sugar in urine, but then in the next hours enough insulin is made to bring bg down to near-normal levels. At this point some symptoms of uncontrolled diabetes may be present intermittently, but not full-blown Ketones/DKA.
Then, something that substantially reduces insulin sensitivity, like an infection or a cold or flu, may "push it over the edge". After bg goes super high, then insulin sensitivity goes down substantially, at which point bg's just keep going up and up, and it goes into full-blown DKA with every classic symptom in the book.
In retrospect, I had some symptoms for at least a year before diagnosis before I went "competely over the edge" at age 14 into full-blown DKA.
Some here may go greatly into detail of antibody tests etc. But if a young kid with no weight problem is spilling glucose into urine and has occasionally high bg's/occasional symptoms of high bg's, I'm not sure why autoimmune tests would have to be run. To me it's obviously T1 diabetes.
I think docs are sometimes reluctant to deal with T1 diabetes if they are not experts in it, and will hesitate to pronounce an actual diagnosis until it trips over into ful DKA.