When I was a child I used to have a dog. When I was a child I had a dog. What is the difference between these sentences?

The first sentence shows that in the past time, you were fond of keeping a dog as your pet. It means, you always kept a dog with you as your pet. But the second one shows that in the past time, you had a dog as a pet but it's not necessary that you always kept a dog with you as your pet.

second one is Past Perfect sentence