Granted, we don't have kids yet, so I may be way off, but I can't imagine spending $100 or more a day on TOP of the dining plan! Especially considering it's just one adult and one child. I think how much you should bring depends on two things. 1.) Will you be eating another meal out each day beyond the 1TS and 1CS you get with the plan? 2.) How important are souveniers?
We usually grab something quick in the room in the morning Muffins and fruit or granola bars), then have a snack at the parks mid-morning, and do a CS for lunch and a TS for dinner. If you do it that way, I would guess you'd need maybe $10-15/day for snacks and drinks above the dining plan. If you decide on a CS for another meal, then I'd say add about $15-20 a day on top of that. TS would obviously be more. You can spend as much or as little on souveneirs as you choose. We've gone and bought many things and we've gone and bought none. Also, don't forget a few $$ a day for tips, like for housekeeping of bell services.
Hope this helps!