This is a known issue with the XAML Parser. Since your PushPin is nested in an Attached Dependency Property (MapExtensions.Children) it won't be part of the same logical tree as the UserControl and as such won't get picked up by FindName in that scope.
Instead, you should manually traverse the visual tree and get the control you need. Luckily, the map control ships with helper methods just for this occasion.
var pushpin = MapExtensions.GetChildren(myMap).OfType<Pushpin>().First(p => p.Name == "m_PushPin");