My boys are 6 & 4 and I don't know that I would spend all the money on Power Wheels now. They WANT another, but I don't know that we'd get the use out of it as we did the other ones and they love their bikes. We have 3 different Power Wheels type vehicles and they have used them a LOT. The two-seater Rhino just wore out after 1.5 years. The quality was not NEARLY as good as the blue Yamaha quad which we have now had over 2 years. The Rhino wheels began to tilt outward, the battery to charge had to be screwed off and on to charge it and the screws didn't line up easily, the tailgate had only a tiny plastic clip to latch it that I could not do easily, let alone the kids. No moving part to the plastic clip, just push close and then weasel/yank open. The design was just not very good and not worth $300 plus! Thankfully we got a great sale on it when we bought it! The blue Yamaha quad has been great quality and convenient, the best of the 3 for sure. A simple screw to open and close the seat to charge the battery, but it lined up well and the design is very functional. Far more durable. The tires are worn out, but like I said, it has gotten a LOT of use. Both these machines go up to 5 mph. Two are not really supposed to go on the quad, but my boys ride it together periodically, motorcycle style and it works just fine. The third one we have is Toys R Us brand that we got when my younger was 2. It has not gotten nearly as much use because it only goes 2.5 mph and doesn't work in the grass (not enough power or tread on the tires). But the boys do enjoy riding it around the driveway and parking it in different "parking spots". With a 7 & 4 year old, it's much more pricey, but the Polaris RZR http://www.livingincomfort.com/bk30042.html
goes 7mph, so faster than the others, and is geared toward 5-10 year olds, so your daughters wouldn't outgrow it so fast. Most of them are geared toward 3-6 year olds (Rhino, quads, jeeps, escalade, etc). It's pretty recent that they've been making the ones geared toward 5-10 year olds. I don't know which others if any go up to the 7mph and are geared toward kids a little older, but the Polaris RZR is.