yes you pull the right side cover off. you might need a new gasket if yours tears. its worth having on hand just in case and less than 10 bucks.
Before I'd drop a ton of $$$, or any $$$ on a jr I'd save up and get a proper racer. The suspension on the jr's is so crappy they don't handle much improvment in speed. Do some freebies like removing the oil injection and jetting properly but before spending money on hop ups that are, at best going to take a 2hp bike to 2.2hp bike.
also you might look at the cobra pw3 and ktm Mini adventure OI. They both have low seat heigths about the same as the JR on the top rung out back. My little guy moved from the pw50 to the pw3 right when he turned 4. Ths suspension makes a huge difference in their riding. Either is a great move once little junior is flying on his JR.