You can work the muscles around your breasts, but since you've had kids..
...it's not likely they're gonna move back up unless you get them lifted with surgery.
One of my sisters had hers done after she had her last child because she's very vain.. *LMAO*
But, I'm pretty particular about my titties too soOo I'll most likely have something done after {if} I have kids since I would breast feed..
I keep mine perky by~
Inclined dumbbells {underhand grip as Pudgy mentioned}
Pec deck machine 3 sets {adjust the seat to 3 different heights}
Cable cross overs
Push ups
At work, you can do isometric exercises by pressing your palms together, place your hands on your desk & do a few sets of pushups.. in between gym visits.