You will make amazing memories on this trip! Since most of the kids are not local (and if you decide against Universal and/or Knott's) you can do a lot of sightseeing for practically free -- like Griffith Observatory, La Brea Tar Pits (museum costs but the actual tar pits are free so you don't need to do museum), LACMA (art museum) offers free kid and chaperone tickets with Next-gen program, etc.
In the Parks, everyone but the tiny one is big enough to run back and forth between the Parks (and he/she will be in a stroller so that's easy), so Parkhoppers would be fine, but with just two days you will see more of each Park if you don't hop. If you're paying for tickets for your entire party $40 extra per person adds up to a huge amount.
I would only bring snacks/sandwiches that fit under your stroller. Or you could bring in a picnic lunch and put in a locker (pretty small and around $7) or just leave it in your car -- you will see lots of folks in the parking structure eating out of their trunks.
No need to bring in water since you can get it free at any counter service restaurant -- often they have tons stacked up so you don't have to wait in line just for water.
Skip the tiny kid's meals and ask if you can order your entree without the side (fries, chips, apples) unless someone really loves those items -- you will save about $2 per entree.
You can also order many items a la carte -- like at the Mexican restaurant on the wharf (DCA) you can order a tamale for about $3.50 and at the pizza place across from the swings (DCA) you can get 2 huge meatballs with red sauce for about $2.50.