I have my first broody right now and I wondered the same thing, as my nesting boxes are over 2 feet off the ground with no ramp that the babies could use. The adults just fly up.
I moved the Mama after the first baby hatched (based on a posting I listed in here somewhere and the wonderful feedback I received) to my smaller coop (someone suggested a dog crate would work nicely too, but I don't have a spare). I used a plastic rectangle 5 gallon bucket and put it on the ground and filled it with wood chips and hay to make a nesting box on the ground.
Once the first baby hatched, Mama, baby, and eggs were moved to the new neting box at night after everyone had gone to roost. Mama has shown the baby where water and food is and they both can easily get in and out of the nesting box.
At some point I may just move the plastic bucket back into my main coop on the ground, or buy some nesting boxes from my local feed store to put on the ground. I'm waiting to see if any other eggs will hatch.
Good Luck!