I'm going with friends who have 2 small kids they don't have a hiking backpack for the kids to sit on their back which is the closest entrance to the waterfalls ??

The main entrance will be the one you want

Entering at the main entrance makes for a fun hike for children. Make sure they have good shoes, there are many steps and the trail can be slippery/muddy. Coming in at the bottom allows the kids to go through a carved tunnel.. cross the bridge that overlooks the entrance.. as you follow the trail you get to walk behind a small waterfall ..climb the spiral staircase carved into the rock! Natural caverns, cathedral & Rainbow Falls. At this time hikers are encouraged to go down the Indian trail back to the parking lot... downhill steep walk with steps.

Really, any entrance will get you to some form of falls. South entrance may be most convenient, but important to note that the main trail is mostly one way traffic, going up, if you want to be along the water. May get the best experience from entering at the main base entrance. Still important to note that the return trail is dirt, partially uphill, and fairly steep steps down the final descent. My 6 and 8 year olds took it well.

The North entrance used to be the best way to go to get to the falls but I think they have closed that off and only have the south entrance by the pool and the bottom entrance on Franklin St now. Going in the entrance at the bottom would be the easiest for you all I think.