When you first start coaching, you may very well be introducing your child to a new experience: sharing the attention of his/her parent with a larger number of other children than he/she is used to. You, on the other hand, are concerned with not showing favoritism.
The Positive Coaching Alliance (PCA) recommends you explain to your child, "I always love you and you are special to me. But when I'm coaching you, I need to treat you like all the other players. And you need to respond to me as your coach, not your mom/dad. Do you think you can do that?"
One idea the PCA recommends is to employ the "coaching hat." Explain to your child that when you're donning the cap, you're coach. When the hat's off, you're back to parent. This can be tricky, especially in your first season. But your child will come to appreciate that he/she is just like all the other kids on the team.