That actually doesn't look like a font, but like custom lettering for the purpose. It is similar to VAG Rounded Bold. <http://store.adobe.com/type/browser/P/P_1130.html>
Note that there is not a single style of lettering used for all fridge magnet kids' letters. For example, we have two different kinds of fridge magnets in our house. One set is in Cooper Black: <http://store.adobe.com/type/browser/F/COOQ/F_COOQ-10005000.html>
The other set is custom lettering, sort of a cross between VAG Rounded and Handel Gothic, with a bit of Insignia.
Then again, we also have some foam letters for playing with in the bath, which stick to the tile and sliding doors when wet. Those are in Tekton Bold:
<http://store.adobe.com/type/browser/F/TEKP/F_TEKP-10012000.html>
As a side note, the letters on our main set of blocks are in ITC Kabel Ultra:
<http://store.adobe.com/type/browser/F/KABQ/F_KABQ-10021000.html>
Cheers,
T