Put no more than one mouse in the radiator and put cap on. In a short time,it will eat all the seeds. If you put two mice in, they may be breed, and compound the problem. Put an odd number of mice in, and you will have one fat mouse, probably female, in a short while. If she is too fat to fit out, she may be pregnant, and you really have a problem. I was given this advice by an "old timer" when I was young and listened all the way to the end of the story, much to the amusement of the onlookers.
The other advice you've gotten here is better, but I think taking the radiator off, turning it upside down, and THEN flushing it would be really good,leaving a cleaner car and engine compartment with more likelihood of getting more stuff out. Also give some thought to backflushing the block from the upper connection.
Have a nice day. Russ