"See and Be Seen" is the way. I'm integrating three week old birds with the rest of my flock every three weeks - as new hatchhlings move into the booder box, and last hatch moves to the grow out pen in the barn, with its attached run. By age five or six weeks, that batch is ready to eat with, sleep with, and explore the pasture with their elder siblings.
Agree with the above. Don't bring the hen to the chicks, place the chicks in their cage out with the adult birds. Make sure they eat together, but from differing sources, just at the same time,in sight of one another. Abundance is a social lubricant - but if the elder flock feels the new birds are taking resources like food from them, integration will be more difficult. Get them used to one another first.
and when you do finally open the cage for the littles, make sure there are plenty of (not dead end) places for them to hide, to run, to explore - its distracting for everyone, and breaking line of sight can rapidly defuse tensions. Contra, a single defensive hen can absolutely command the whole space in a narrow, barren run. That tends to end badly.