@tjmcki I think the key here is don't tell them. I have never told my youngest the circumstances of her birth, all she needed to know she was loved from the first moment she was born and the two of them are very close to each other.
I'm sure bonding with the father was the only reason, probably not even the biggest reason. I found myself in a similar situation, although not intentionally, became pregnant after I had already separated with my first child's father. He was only 9 months old so not much of a relationship yet with his father but I knew he had a challenging road ahead of him and thought it would be easier with a little brother or sister. It was definitely tough being a single parent whilst pregnant and a toddler, honestly those early years are a blur, but I am eternally grateful that they have each other, as well as me of course!