There's just too much wrong with this to really enjoy it. It would be far better as a stand-alone than a spinoff.
To me, Pacific Rim is about huge lumbering steel giants punching huge lumbering meat giants. The best part about the fights in the first movie was the sense of scale and most importantly weight. All this is immediately lost when you fight mostly in the desert and your main mech routinely tumbles along the floor after getting punched, when he should drop like a sack of flour. That only gets worst with the confusing biomech that does literal backflips.
The omission (mostly) of weapons also highlights this, because fights get turned into a MMA brawl with kicks and grapples and everything.

Characters were mostly OK, with the exception of Shane. [spoiler] He was just evil for evils sake. Why not have the kids pilot and just put two soldiers with guns to their heads into the cockpit with them? Instead he fries the brains of the only guy who can repair and maintain yaegers in the apocalypse. Why shoot rockets at the "only yaeger in the black", AFTER he has basically killed said engineer? Just wait for the power-cell to run out or the kids to get supplies. Why have the kids killed after they leave exactly? The list is goes on, but the point is he's a cartoonishly evil, poor excuse for an antagonist. Shoulda gotten stomped.[/spoiler]

The inclusion of the small Kaiju somehow made the big Kaijus way less terrifying, as the complete helplessness the characters had against them meant that any sense of danger was gone as soon as they got into the mech.

I won't even go into the whole "boy" situation. At best I'd call the character obsolete.

Finally focusing on kids is always a mistake. I didn't even want to see kid-protagonist when I was a child. Funnily enough, the only cool fight in the show is the one with their parents in EP1. At that point I didn't mind the fast mech movement too much though. At least it looked cool.