I still have this problem and my kids are 6 1/2, 5 and 7 mos. We let the big kids play outside, watch a movie, play Wii in the basement, read, color or any "quiet time" activities. For us, the third is being dragged EVERYWHERE. It seems like just when I get him down for a nap, 30 mins later I'm waking him up and getting him back into a carseat to drive to preschool pickup, a girl scout meeting, etc.
Even tho your kids are so small they need to learn the meaning of "quiet time"...using sign language "shhh" finger to your lips.
Keep "noisy toys" in a box you can close up. When it's "quiet time" break out the coloring books, arts n crafts or play outside with a monitor nearby.
We have used a sound machine EVERY day since bringing home baby #3. We use the "static" sound. In the beginning it was VERY loud (just like the baby was in utero). Eventually we'd make it a bit quieter. At nights it was so loud we'd close the baby's door and listen on the monitor. It definitely helped drown out the other "house noises" during the day.
Also, the SWING! We had a bassinet style swing that cradled the baby. This was a Godsend to help the baby nap for more than a brief catnap.
Sound machine. Swing on. Swaddle the baby. Close the door. Get the monitor and play outside or in the basement with the big kids.
Good luck