I photograph many combinations of families with dogs, kids with dogs, even small babies with dogs and I am always asked for recommendations on how and when to add pets to family portraits. We often have parents with newborns that want to bring a dog!
Some families just know when the time is right, but consider these three things before scheduling your photo session.
1. Is the baby sitting up by himself? Because if he's not, and we're trying to get an artistic photograph of the two of them, the dog will always wins the lion's share of the attention. A baby under six to seven months is difficult to position with a pet because he can only lay flat (or be artificially propped up) and the viewer's eye will immeditately go to the dog.
2. Consider the relationship between the baby/and or child and your pet. For one thing, many pets do not enjoy being mauled by a toddler, no matter how cute it looks. They may suffer quietly and even act like they're okay with it but they're really just trying to please. A kids photo session at our studio often includes a pet, but usually when the child is old enough to know how to approach gently and sit still. Or we make it a family portrait and get everyone in on the act so that parents or older siblings are close enough to run interference.
3. Attention span. Here is where the baby and the pet may be on equal footing. They both tend to be fidgety and easily distracted. A quiet park (NOT a dog park) or a sheltered back yard can work. Any place where they both feel safe and can listen and take a little direction.
In general, age 3 is usually an easy time to capture the bond and personality of both subjects. A rare exception sometimes occurs with certain larger breeds that just seem okay with anything and everything, but it's usually better to be safe and wait for them to start interacting on their own. The results can be pure magic.