Thank you, Marta!
If you look at images of boys that are a bit older - say 5 -7 yrs - their pants are still that "blousey" cut like little boys. You also can, if you like, make them with a fly front as he becomes more able to work with a button fly. So you can plan to use the trousers perhaps longer that the rest of the pattern pieces.
Great idea! My boy will be 7 this summer, but due to a limb difference wears a size 4 pants and is just 41" tall.
Wait! You mean they eventually start to use the fly with all the button holes I painstakingly sewed? I swear both my older boys shimmy the trousers down and never bother.