Playing Nice by J.P. Delaney

Pete Riley and his partner, Maddie, are stunned to discover that two-year-old Theo is not their biological child. Theo is actually the son of Miles and Lucy Lambert, who have been raising Pete and Maddie's son, David. The boys were switched sometime after their births, when both where transferred to a different hospital because they were premature. Miles explains that he would like both families to get to know one another and be in each other's lives. Soon, however, Pete and Maddie realize that Miles is controlling and is very keen to play a big role in Theo's life, since there are a lot of things David can't do because of developmental issues. Their lives become a nightmare as they face financial ruin and the fact that they might lose Theo to a monster who resorts to violence when he doesn't get what we wants. A novel of domestic suspense that's hard to put down.