Is this a trip of a lifetime, or likely to be repeated?
I went as a single mom when my daughter was 4 and my mom ended up tagging along. It was good to have extra hands when I (or your older girls) wanted to do something non-toddler style. She did get tired easily, which made us return to hotels for an early break and bedtime, and we missed rides that an older kid may enjoy. If it's a trip of a lifetime, my personal advice would be to wait.
That said, I found that first trip to be so easy to do, found value for the money, and using this site and others found ways to fit it into a budget, so what I thought was a once or twice trip has now turned out to be more often because we loved it so much. We were able to get back to hotels and nap easily, and then go back to parks. Characters and the pool were a huge hit and will be for all your ages. There's so much flexibility to do your trip, your way.
Disney has been a great place for my daughter and I; she's now 14 and we've just started thinking about our next trip. Hope you all find the same Disney love we did!