I thought it wasn't believable as this setting is based on our medieval period, when people were considered adults as soon as they hit puberty. Tommen is, what, 15 ? Margery shouldn't have any qualms about anything sexual with him. I thought her teasing him, followed by a kiss on the forehead was a decision on the part of the producers to avoid controversy because Americans would be all like "oh an adult kissing a teenager ! PEDOPHILIA".
Yeah,
big fucking deal.
Yeah, but you're projecting your belief of producer concerns of controversy on a scene that would not have necessarily benefited from the change you're suggesting let alone been relevant to the characters. Just because it's medieval fantasy, and the lore is pretty flexible with sexual relations with children, doesn't mean it's relevant to this particular scene.
Think of the characters involved, their age, their mannerisms, and their goals. Tommen is just a boy. A boy that I'm sure at his age has sexual thoughts. But still a boy. One seemingly more concerned about his cat Sir Pounce than the hot chick in front of him who is also kinda giving him goose bumps because girls. Tommen is also displayed as relatively level headed, while also a bit shy and concerned about his situation.
Margaery is a morally balanced individual who desires to become queen but not at the expense of others and certainly not her own life. She's going to be trapped in King's Landing without protection from the other Tyrells while also being hunted by Cersei who'll be watching over Tommen like a hawk.
Hypothetically, Margaery's advances against Tommen, a clearly sexually oblivious child, spook or confuse him. Loose lips sink ships. He says the wrong thing, or no longer feels comfortable with her, and its her head on a stick. Tommen doesn't just have Margaery in his ear. He has Cersei. He has Tywin. He has the lords. He has a whole bunch of people telling him all kinds of shit as he struggles to now become king, after his sociopath older brother was murdered for the same thing.
Margaery seems to pretty accurately read Tommen's innocence and insecurities that come with his age. Listen to the dialogue, the way she tells him to tell her a secret, lets him open up a little about Joffrey, and supports his belief that his brother was cruel while letting him explain his own personality a little. Margaery does want to become queen, but the only way that's going to work in the current climate at King's Landing is to get the support of Tommen, which isn't going to happen if she throws herself on him sexually.
She sees that Tommen, the soon-to-be child king, probably just needs a friend. And she, a person that wants to get into power with Tommen, has the perfect opening in befriending him. I assume she has many more months to work with Tommen, all the while avoiding Cersei poisoning/assassinating her. No point rushing in just yet, not when she barely knows him and him her.
EDIT: The TLDR would be that you're right in that the kiss was very deliberate, but I don't think for the reasons you suggest. It wasn't an innocent kiss because producers fear backlash, but an innocent kiss because Margaery read Tommen's character and made the right call for the moment. Reny wanted to fuck/get fucked by Loris. Joffrey wanted to use hookers as target practice. Tommen just wants to play with kittens. Margaery has consistently shaped her interactions with all three of these characters based on their personality.