i found this reallllllllllllly hard. i had a newborn and 19 month old...and a toddler too to try and keep quiet whilst baby slept. I still find it hard......
Whilst baby slept i always stayed with the girls...played with them, engaged them in a puzzle, colouring in etc - an activity that would last a while. Always, always did morning tea outside, let them have a run, scream around on the swings etc...... IT was also our TV time.....it was the one time in the day i would put the tele on and that way the girls would be reasonably quiet whilst baby slept.
Since having my #4, my #3 was 20 months old at the time. I found her the loudest and the most difficult to quieten down.....it just takes time...and tolerance unfortuantly from baby...

GL.