I'd start with a straight forward arc welder (stick welder) ... much cheaper than mig. You will learn good technique. Loads of how-to videos on youtube
If Junior gets fed up ... not lost a lot of money.
A single phase welder in the 180amp range will do you up to 4mm rods Will run off a 13A plug on all but highest settings.
( I added a 16A socket to my garage to allow full power use)