To consider anything other than commercially processed rations to be nutritionally unsound “treats” is somewhat misguided, just as is the notion that optimal health could possibly be obtained on a diet consisting exclusively of that commercially processed feed.
When chicks are raised by a mother hen and given opportunity to range, they’re all out there together by day two, and they aren’t foraging for grower pellets. Some of the most important early foods young, brooder raised chicks should be given are clumps of greens - chickweed, dandelions, untreated grass - roots, dirt ball and all, to help establish the gut bacteria.  It makes sad sense that if grower ration was the only food a bird had known for its entire life it might be a little hesitant to try anything new. 
Don‘t give up!  Be patient and continue to offer your girls fresh, real foods.  And on the watermelon- leave a nice layer of red flesh on intact “smiles“ of rind, large enough to stabilize like a plate, and they will work for hours pecking off every bit of flesh.