Formulaically, on paper, this should be a slam dunk. They took a very popular, well-written book series, and are sticking to it so hard that you recognize a lot of dialogue from the books.
The main issue is the delivery. The lines are just... dry. The delivery is so, so boring. I know they're kids, but we've seen what kids are capable of through other shows like Stranger Things. The whole "shipping Medusa's head to Olympus" doesn't make a whole lot of sense because the emotion/passion is so underwhelming. Sure, Rick Riordan's dialogue provides a certain level of sass, but you can't just rely on the pure dialogue itself. You need believable delivery - and if you need to stray from Riordan's specific text lines to make it feel more natural and give actors room to breathe/get excited about things, you do it.
I'm thinking, especially, about smaller, more impactful moments - like the "you drool while you sleep" line. I don't know what I was expecting, but it certainly wasn't somebody delivering that line less like an off-hand remark, and more like a robot reading a script. I just don't think these kids were adequately prepared to deliver the humor/passion/feeling that made the books so special--and nor were the adult actors.
TLDR: books good; idea good; execution/acting bad.