Even if on paper it looked like I could save a few bucks, I wouldn't do the dining plan with my son (he's 5). He's picky and doesn't want to eat one day, and the next day he's ravenous for nothing but hamburgers or something. The plan seems like a good idea if you're doing a lot of character meals, but even then it won't save you more than a few dollars--and if you skip even one or two TS meals, due to illness or just not being hungry, you've lost money.
Quick service meals are pretty big in WDW, and one meal, or one meal plus an extra side, is often more than enough for him and I to share.
I go into character meals with the mindset that I will not get my money's worth for what my son eats. He's too excited about the characters to eat, usually. But it's worth it for the experience and not having to stand in line in a park to meet characters, so I always do them.
I love the refillable mugs because I easily save money on coffee and my son loves lemonade, and we spend a lot of time hanging at the resort. For your child, it depends what he likes to drink--milk and juice aren't available, only soda/iced or hot tea/lemonade/coffee/hot chocolate. If my kiddo didn't drink buckets of lemonade on vacation there would be no value in the mug for him.