You are on the right track, hiring a pro! They have the experience and the know-how to get the job done right. Way to go, Mom/Dad!
BUT... there are a few small things you can do, to help things go a little more smoothly for your youngster (and your photographer, and YOU!) that I have learned from both being a photographer for 10 years, and a mother for 13 years.
5 TIPS FOR PARENTS ON HOW TO GET TODDLERS TO SIT STILL FOR PORTRAITS
2. Feed them before you go to your session, but also bring a small, tidy snack to coax smiles. Smarties are a favorite. I don't recommend cookies or crackers, especially with younger children; they tend to get gooey! I like for the parent to be the one to bring the treat, as they know what their child likes and if they have allergies. I use these as a last resort, in most sessions. And as a reward for a job well done, afterwards. At the parents discretion, of course.
3. Dress them comfortably. I know that we all want to see our kids dolled up, but make sure the clothing is comfortable. Especially with younger kids, their comfort level can be a distraction from smiles. Check out my Facebook Page for seasonal tips on clothing and coordinating the family for portraits.
4. Ask for direction. A lot of parents either overshadow the photographer's natural skill with coaxing kids to smile by jumping in front of the camera and micromanaging, or sit there in the sidelines silently and get frustrated because their toddler is a wreck. Ask what they need from you; after all, you are Mom/Dad, and you know how your child will respond to certain things. They may need you to stand behind them, because your little one will not take his eyes off of Mommy. They may need you to use the treats that you packed, so they can keep their hands free. They may need you out of the way. Your little one may need you close to feel comfortable. the key here is communication.
*And if you know something frightens your child, let the photographer know so they don't accidentally pull out a clown doll to try for a chuckle, and end up with screams!
**Not that I would use a clown doll; clowns horrify me.
5. Make it fun! Don't stress about the "perfect portrait" because the one that is perfect is the one of your child that makes you smile or tear up a little. The one that speaks to your heart. not the one that was copied from your Pinterest board! Genuine moments always make for better pictures.