I like 609, that is pretty unique and you likely won't conflict with someone else with that one.
You could do something I would do. Take the two kids b-days, add them together, and then average them out and make that your number. So if the oldest's b-day is 5-10 and the youngest is 7-28 do this:
510+728 = 1238 , then 1238/2 = 619 and there's your number.

Of course this fucks up if one of them has a birthday that is in October-November becuase it could give you a four digit number.
(Yes, my brain does weird things sometimes..).