It depends on how long you're going and what that money is for. Is that for food, or just for souvenirs and other extras like that?
If you plan to eat in the parks, you should plan on about $40-75 per person, per day - but that depends on where you eat and how many meals you eat in the parks. We often bring our own breakfast foods to eat in the hotel. If you stick to quick-service places, then it'll be on the lower end of this. If any of those people are kids, then it might be even lower.
No one can advise you on souvenir spending though. There are things for as little as $5 and things that cost hundreds of dollars. So it depends on what you would buy, and how much you would buy. We can't predict that.
If you are also wondering about ticket and hotel costs, then that depends on lots of other factors - when you go, how long you go, where you stay, etc. Since you said "spending money," I assume these things aren't included in what you're asking about.