Currently the "child" handle is disabled for all adopted children (adopted in or out). It should be possible to record in the pedigree at least the children of adopted out individuals, since the relationship is genetically relevant. However, there's no good reason to prevent a full family (including children of adopted in members) from being recorded.
This is not exactly what happens, adopted in children have all the handles as far as I can tell (while adopted out have no child or relationship handles).
This comes from the assumption that we have no idea what happens with adopted out children after the adoption, but I guess it is a too restrictive assumption, so we should just have all the handles period (I guess this is in the same category with not checking genders for compatibility, etc)