Hi Richard.
This can be done on Tyros 4, but because the keyboard has extremely limited on board memory wav/uvn memory,
you'll likely need to purchase & install optional Tyros 4 expansion memory module:

Yamaha FL512M 512 MB Expansion Board
or
Yamaha FL1024M 1GB Memory Expansion

You will then be able to import a .wav file of the children laughing into the T4 expansion memory:

On The Tyros 4 Keyboard: Voice Creator Button > Expansion Module (E button) > Import Wav (H button)
* see page 17-19 of Tyros 4 Ref Manual for more details

When the Wav file is imported, it's automatically converted to UVN by the Tyros 4 keyboard itself, and then stored in expansion module flash memory.
You can then use the T4 MultiPad Creator feature to trigger the UVN file to playback from any one of the 4 MultiPad buttons.

Digital Recording Button > Multipad Creator

Here's a related thread on this topic.

Hope this provided the answers you were looking for.

Scott