Both the Telerik MapEllipse and the native WPF Ellipse elements do not support setting elements inside of them.
Instead, what comes to my mind, would be to place the MapPinPoint instance on top of the ellipse. To do so, retrieve the location of the ellipse using the MapLayer.GetLocation method and calculate the location on which the MapPinPoint is going to be placed on top of the ellipse. Then, based on the chosen position, you could set a new location using the MapLayer.SetLocation method. Finally, to display it on the RadMap control, add it to the Items collection of the InformationLayer instance (I assume that this layer is used since the MapPinPoint element is utilized).
I have prepared a sample project for you to test regarding this inquiry.