You are right about Shannon not being bright.. but she 100% planned her second child and I find the way she did it really manipulative and disgusting. She kept mentioning having another child, it was made clear to her he did not want another one. Charlie said himself he thought he could trust her as they were in a relationship and clearly couldn’t. Also Theo was not a handful from what we saw
he was a normal toddler and Shannon just couldn’t cope. Another reason why it made no sense to have another child. Men can be blamed for a lot but in this case it was Shannon being conniving.