Alright, we could do something a bit risque and saw the father (Mary's employer) had some mistress but soon broke it off. We could say the mistress took the child in hopes of making her ex suffer but perhaps to also get some attention she feels she's missing out on? It could give us some colorful places to inspect which might be a bit comedic for John and Mary? Now Holmes might be regretting his choice to sit the case out. We could ave the kidnapper want to even sell the child maybe to add another layer of confusion. At the same time it depends how long we want the case to span.