If you're only there for a day, I'm not sure I would bother with Disney Studios. The car stunt show is very good (our kids loved it) but it will take up a couple of hours by the time you've trekked over there and watched the show. If your kids like thrill rides there are a couple at the Studios (Twilight Zone, which is a lift-drop, and Rock 'n' Rollercoaster, which is a rollercoaster in the dark, but not dissimilar to Space Mountain in Disneyland), but the smaller rides had horrendous queues when we were there at half term. If you only have one day, I would stick with Disneyland itself - there's plenty there to keep you busy. The good rides at Disneyland are Big Thunder Mountain, Space Mountain, Pirates of the Caribbean, Indiana Jones, Star Tours, though some have height restrictions. You should also do some of the smaller rides, like the teacups, and check out places like the Swiss Family Robinson Treehouse. The dragon already mentioned is well worth a visit, and something most visitors are not aware of.
Check out which rides have 'fast passes' and make use of them, so you have less queuing. The steam train (already mentioned) is a nice way of seeing the park, and there are various shows during the day (good time to get on popular rides if you don't want to watch) and chances to meet characters. We did a ride on the paddle steamer which was nice as well and gives you a different view of the park.
The food is horribly expensive and, for the most part, not that good (burger-type meals). We took a picnic each day, however they don't allow you to picnic in the park, and there isn't anywhere you can really sit down, unless it's on a bench somewhere (no grassy areas). There is a picnic area, but it's outside the park; we went there each day, allowing us to get away from the tweeness of it all for a bit, but I wouldn't have done this if we'd only gone for one day. Take water bottles with you, as there are water fountains dotted around. Drinks are expensive.
If you're not staying in the Disney hotels, we stayed at a great campsite a few miles away, where we rented a little chalet. This also helped cut down on the cost of the whole trip and meant we could leave the Disney-fied atmosphere behind.