I agree with the hide and seek or chase game. I had a rotty for many years and I would throw the ball and she'd look at me with the "Now go get it dummy" look. But if she had a toy and wanted to play she'd run away all day, give up the ball on command and fetch it, but only if I ran chased after her.
But she loved to chew, tug and chase.
Edit: I know it sounds silly but is your kid involved in the feeding / reward process ?
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