On Completion of Nanny Knows Best, this event is unlocked.
If you can complete the Hobby within seven days, you will earn a baby Jumper for your Infant Sims. You will also receive the Baby Play Mat.
To get started you'll need a baby and a Xylophone. You can get the Xylophone from the Promotions R Us Store. Once you have purchased one (2,000 Simoleans), you can then get it from the Home Store > Inventory > Hobby and Career Items.
You can get an Adult Sim to pick up a baby by tapping the baby and selecting Cry, the Adult should pick the baby up, then select the adult and choose, let infant crawl. Then just tap the Xylophone.