DS1 takes after me and has curly hair, it's fantastic but it's definitely not as easy as DS2's mostly straight hair!
Both DS and I shampoo our hair, I've never found the curly girl thing works for me because I just get greasy weighed-down roots (that could be user error...) and I've always found DS's hair needed a proper 'clean' because, well, kids get mucky
so apologies for not being able to help with that aspect. DS is 11 so he does this all himself now, but when he was little I always conditioned it after shampooing, then combed it through with a wide-toothed comb while he was still in the bath. Then when he got out I'd run my fingers through it while it was still sopping wet, flip his head forward, and scrunch it with a towel to take some moisture out and define the curls. I do the same with mine, except then I put in a bit of curl cream, DS doesn't bother with that. Then either air dry or diffuser. I think the key is not not separate the strands too much while it's wet, so the curls clump together.
The frizz I find comes from sleeping on it, so if it needs it we spritz it with water/detangler/leave-in conditioner in the morning, then scrunch. DS doesn't really use any products in his hair, but I find a little bit of oil or serum helps mine be a bit more defined. His does get frizzy the more days he goes between washing it, probably because again, kids will be kids and he isn't particularly careful or precious about his hair! It's wild and mad and crazy, just like him, really!