For day time, could you set up a temporary pen in the yard somewhere, something that the chicks can't squeeze through? This way the older birds can get a good look at them in the space they frequent for most of the day.
In the evening, continue having them in the brooding space in the coop.
After a week or so, if the older birds aren't too overly interested in the chicks, you can lift the pen fencing up with a few bricks to make chick-sized spaces to let the chicks dart in and out for safety. Might need to supervise these sessions as I don't know if you feel comfortable having young chicks free ranging.
If older birds don't fuss with chicks too much, in another week I'd then try moving them into the coop proper. Do you have a separate roost the chicks can use to minimize pecking from the other birds?