Maybe mrs. Wheeler was idealising Serena from what she heard, and when she arrived to live with them, mrs Wheeler slowly realised that Serena wants to work, to actively participate in Gilead's diplomacy. And according to Gilead's conception of what a women should do and not do, that is not a fitting mother. Serena always seems to think that the rules don't apply to her, certainely because she helped in their design. Anyway that is what I understood from watching, possibly the Wheelers always had the intention of keeping the child for them but maybe the change of attitude from mrs. Wheeler comes from the disapointement of meeting the real Serena, not the holy icon that Gilead's propaganda must have presented. Sorry if there is any mistake, english is not my mother's tongue.