Well, I would remove the Monday hatching chick... and slip them ALL under her when the last one has hatched.
My oldest successful adoption was 6 days old. Yes they need to be under mum hearing her clucks and she learns their peeps. In this way they learn how to listen to and follow her and recognize her voice. The older the chicks get, if they have been under a lamp... they may not realize mom is the warm safe place. The older chicks are already adventuring out on their own and have zero interest in listening to a hen. But have carried out dozens of successful adoptions.
For future reference... If you'r gonna let a hen sit... collect ALL the eggs you want her to hatch and place under her at one time. In nature, staggered hatches are a death sentence for either the early or late hatchers depending on what mums' instincts tell her. But the early hatching may make her leave the nest to search out food for her babies. Or she stay stuck to the nest and the early hatcher starve/dehydrate. She usually stays stuck to the nest for a couple days letting hatch finish up. I place feed and water nearby so chicks and mom can access safely, collecting at nigh because chickens don't eat in the dark and rats do.
If you take the first one and the second one doesn't hatch you can still put the little's under her and remove the unhatched one. Adding in other chicks once hatch has taken place and mom has them out of the box would not be recommended in my experience.
In regards to eggs getting broke... earlier hatching CHICKS inside my incubator have NEVER cracked an egg... despite the heavy amount of soccer ball that sometimes gets played in there. It's always scary though... wondering how eggs that just got knocked to kingdom come will still be able to pip the right location and not be twisted!
Yes, I do adoptions early in the dark evening, giving as many hours as possible for her to hear and feel them moving under her. Tuck every one in keep it dark and listen for activity. I come back out a couple times to make sure no one accidentally got out and can't find their way back under mom. I have found many a cold, barely living chicks. I come out EARLY in the morning to double check this, before and after letting the flock out of the coop. Blocking so the young chicks can't get out of the box the first day can help this. If another flock member causes chaos just out of curiosity chicks are quickly disoriented and mom has to decide to go for the other chick or stay with the ones under her. Very often she stays where she is... at least at first. Broody's are not without challenge.

But how special it is to see them leading their little families around!
Happy hatching!

