Yes, we did the falls with our kids years ago and they really enjoyed it. Just be sure to hold hands with your 5 year old, the rocks can be slippery, be sure to also wear water shoes or an old pair of sneakers. I think the whole family will enjoy it, have fun!
Hi -- Yes, with ages of the children you listed it should be ok. You may have to carry the 5 yr. old in certain places.
I suggest, if time is a factor, to take a taxi from the pier. Distance to falls is about 5 miles.
The falls is like a park. Has dressing rooms and the like. Also, good observation decks to observe people climbing the falls and from which to take photos.