Lovely quilt. I'm a little confused about your border delimma. Are you saying once you put the pieced border on you think you will cut off one of the triangles? Sometimes that can be fixed by just taking in a seam here and there.
Regarding quilting... You say it is for your 3 year old son but is this going to be a utility quilt or a wall hanging? If utility, you may want to reconsider and machine quilt just for the added strength, tots are tough on stuff and if this becomes a favorite binkie it will be hauled around with him everywhere and probably converted into a super-hero's cape on occasion :lol: .
If that is an option I liked Blue's idea of an all over meander (aka stipple). The quilt's peicing is what you want to shine and the meander won't take away from that. If you are bound and determined to hand quilt I would do a single 1/4" echo inside every triangle in the quilt and again in the border, IOW quilt a 1/4" from every seam in the piecing.