You can set the ChildForm property to the assembly qualified name of the type. We generally only use the name of the type like this:Namespace.FormTypeName
but if you change it to this:
Namespace.FormTypeName, AssemblyName, Version=X.0.0.0, Culture=neutral, PublicKeyToken=null
which is the assembly qualified name, then it will be able to find it. You can get the assembly qualified name by typing this in the watch window while debugging:
typeof(YourFormType).AssemblyQualifiedName