is letchworth state park doable with little kids?
Like most National Parks (I know this is a State Park, but it can easily pass as a NP), it really depends what exactly you want to do there, i.e. how much hiking, if any.
But personally, I think you can do it. You can see most (perhaps all?) of the main viewpoints without much walking from the car.
There's lots of picnic areas and even a nice playground FWIW.
Obviously if you have a baby, a baby carrier would always be very helpful.