It's been great photographing portraits of children for my client's, but sometimes what is the best time for the parent is not always the best time for the child. I will always ask parents to plan their portrait after their nap and eating times. That's when children are usually the happiest and adventurous. Take a look at your child's daily behavior. When are they the most lively, and when are they the most fussy. As adults, we can adapt easily to anytime needed for a portrait, but for a child it is very different. Whether if is is for a child's portrait, or a family portrait, try to plan the time schedule around the child's daily pattern for the best results. Now, sometimes that might not be the best for beautiful late afternoon light, but there are times that you can prepare your very young child in advance for that time period by taking the outside at that time to be acquainted to it.
So, plan around your child's timeline and see what your child can tolerate ahead of time by doing something with them outside during the preferred time for your upcoming portrait.