The regular dining plan for 2016 is $63.70/night. That's 1 snack (average: $5), 1 QS meal (average: $18) and 1 TS meal (average: $37). Throw the other $3.70 towards the mug to get your $63.70. Child plan costs $22.85/night. Snack is $5, QS and TS both end up around $8.
So to be "worth it," you need the total cost for 1 adult+1 child to be more than $90. Current SOA pricing is $72/43 for category 2 seating, and $64/$38 for category 3 seating. Obviously, stellar deal on the child side. On the adult side, it's about break-even. Together, the savings on the child make it a good deal when booking both if you don't have any more expensive TS meals you want to spend that adult credit on.