Useful advice, Mrs. K! I am considering doing the same for my broody who I think only has one viable egg left.Get the chicks a little cold - the point about the chicks running away is valid. The chicks need to be a bit chilled, not life threatening, but in serious need of a good warm up. Do it in the dark, let the broody listen to their distressed peeping for several minutes. Then in the dark - slip them under her, they should burrow in tight. If they do so, she will take them.
Often times people blame the hen, when really it is the chick that is not making the bond. If they get too old, they won't see her as a warm place. I would do it tonight. I would not worry about the egg count or chick count, hens can't count.
Mrs K