Do it. I just made a frame bag with my 4yo son for his bike. It was bloody difficult, but mainly due to involment from said 4yo I think.
Lessons learned:
Simpler is better
Think and plan ahead
Don't stitch through too many layers
My son can not yet be allowed to control sewing machine speed.
That's pretty awesome, cheers drøn!
Might have a go at the frame bag as well.