Form handling for generic class in Play Framework 2.3.x

I have a set of model classes on the basis of a sealed trait like the following:

sealed trait Base {
   def baseValue: String
}

case class ChildA(
   baseValue: String
   childAValue: Int
) extends Base

case class ChildB(
   baseValue: String
   childBValue: String
) extends Base

How do I handle the form submission properly, i.e. how can I define a Form that yields objects of type Base based on the given input data. I am expecting an extra field $type that holds the class type information for the specific subtype. Ideally, the form would also handle unknown types with a respective form error.