Question: The center I volunteer for has received 22 Window 7 Professional PCs and a printer, all are networked to a 24 port D-Link switch.

What software could I use to configure multiple PCs at the same time. For example install software, configure system settings and also restrict access to certain websites (it is a children center).

The software has to be open source as we do not have any funds to purchase any licenses.

Answer: 
Active Directory running on Windows Server would be your best bet. Failing that there are a mass of open source options for doing a lot of this stuff.

Microsoft offers discounts for registered non-profits, and if you can make friends with someone at Microsoft, you can probably get them to give you all sorts of good stuff.
Well you've basically got 2 problems there. Setting up the computers identically and restricting web access. My votes are for [FOG](http://www.fogproject.org/) to image all of the computers to a standard image and [OpenDNS](http://www.opendns.com/start/) to control web filtering.

As for keeping the kids from changing the settings, that's a bit trickier. You best bet for free is to try and lockdown the image, removing things like My computer, search box, start menu, ctrl+alt+del access. Locking down a computer is non-trivial but the harder you make it the less likely things will be changed accidentally.