Old style animation in WPF

Hello,
I-m just starting developing in WPF and Expression Blend. It-s quite
different from Windows Forms. Anyway i'm making my way through. I'm
just wondering how to do the following thing:

I have a control where I put shapes doing Children.Add(...) . I want
to manage these shapes at certain time intervals. What it has to
happen is: set up a timer, say every 100 msecs; when the timer ticks,
I want the callback to modify those shapes. I I do it using a plain
old .NET Timer object, it will complain about thread access. How can I
work out around this??