As long as everything works, nothing needs to be upgraded.
Some grip shifters are very stiff, hard for an 8 year old to turn. If so, I'd look into simple thumb shifters.
If it is early 1990s, then I would replace consumables: tires, brake pads, cables, lube chain and service bottom bracket, headset and wheel hubs.
Its the top tube that looks curved, that is not unusual for kids bikes.
Kid's bikes were made to meet a price point, and light weight wasn't typically a critical design feature. Lots of heavier steel parts on the bike: chain rings, seat post, most likely handlebars and stem, nutted axles, etc. No suspension is a plus IMHO.
If RD is Shimano:
https://www.amazon.com/lundeng-Shima...-goods&sr=1-10