As a photographer I often have to contend with snacking during a photo session - when to give, when not to give, what to give and if it is transactional. The best advice I can offer it to make sure they were fed recently so they aren't so hungry. As a parent I know they are conditioned to ask for snacks every 30" however!
What's transactional? Well if a child isn't old enough to understand a bribe, it's better not to present it that way. "If you do this, you can have this." Instead you get a child who is frustrated and miffed at how stingy you are with the treats or snacks. Disaster.
What not to give your kids - anything that gets stuck in their teeth. Such a sweet smile except for the orange goo of the cheddar goldfish, or the raisin sludge amongst those pearly whites. Try green grapes instead.
Some snacks can be very cute to include as part of a game. "Lie on the floor and I'll pretend you are a fish and I'll drop the bait in your mouth." Lots of anticipation and eye contact.
Snacks as props are another fun element. In the summertime it's festive to have a wedge of watermelon or an ice cream cone (spare change of clothes alert.)
Finally, a word about lollipops and other sugary treats. Never give one until the session is almost over. You don't want a green mouth preserved for eternity, nor a hyped up little one headed for a sugar crash. However a treat at the end can really get your small people excited for a last hurrah of joy and then they'll go home to a well deserved nap, or maybe you will.