Hi! Welcome to the frog forum!!

Frogs should only be fed live food and that food should never be wild caught. Pet stores usually sell flightless livefruit flies and/or pinhead sized crickets. Both of which should be small enough for the frogs.

The toothpick becomes unnecessary after that but you should be using blunt forceps to feed the frogs OR you can try bowl feeding. Get a small glass bowl and put the dusted feeders inside. Unfortunately the fruit flies can climb up the glass and escape. The crickets, however, cannot scale the surface and are usually not smart enough to jump.

Babies and Juveniles must be fed EVERY DAY

Calcium powder should be used three times a week and multivitamin powder once a week

How big is the size difference between the sick frog and the others? They may be bullying and/or intimidating him into not eating

try using a low, escape proof container filled no higher than the frogs chin with the pedialyte solution. It should be low enough that the frog cannot stay on the glass, but not too constricting. Escape proof but with proper ventilation

They do not require UVA/UVB exposure although some do argue that it improves color and behavior.

you should not be transporting or handling the frogs any more than is absolutely necessary. This stresses the frogs a great deal.

the frogs turn brown for different reasons including humidity and temperature i would not worry if the frog turns brown.

Finally, the frogs should be kept in a quarantine enclosure for 3-4 weeks before being placed in their permanent home. The setup of the quarantine tank should include:
-damp unprinted paper towels as substrate.
-a water dish that should not be filled higher than the frog's chin.
-a place to hide
-a few things for them to climb.

here is a care sheet as well. GOOD LUCK! The basic care of the green tree frog

whoops almost forgot. PICS! They are important. Both of the sick frog and the enclosure please