It's great that your players are being creative and thinking outside the box! But because we're the the DM, let's just help them fit their idea inside the game rules that you all have agreed to follow.
The rules for Alter Self state:
You grow claws, fangs, spines, horns, or a different natural
weapon of your choice. Your unarmed strikes deal 1d6 bludgeoning,
piercing, or slashing damage, as appropriate to the natural weapon you
A tentacle is just a different natural weapon, and the way they want to use it is just flavor.
When your players attack with a sword, why can't they just say they cut their opponents head off? Because they are in a fight. Of course your player is able to cut people's heads off in a single strike: he is a skilled fighter with years of training that have killed countless people bla bla bla. But why don't they? Well, they would have, had his enemy not dodged his sword at the last second, making what would have been a fatal blow a mere scratch, explaining why he, in game, only took 4 points of damage.
I'm assuming you and your players have agreed that the tentacle is small enough to fit inside a human's mouth. If not, whatever, maybe its squishy.
Describe the fight, how your players are trying to kill their opponents, but how the circumstances of the fight don't let them. Maybe they did get the tentacle inside the enemy's throat, but they were pushed right before they could smash any organs. Still, the opponent is left coughing blood. Maybe the enemy closed his mouth at just the right time, and the player ended up hitting him in the mouth, breaking some teeth.
At one time, describe how the player went for the eye, and successfully ripped it off. Just don't let them succeed twice, or you'll have given your players a free blind without any spells or... or do it if you want to, if it feels right for your group and your style of play.
And when they finally give that final strike, sure, describe how they put the tentacle inside the enemy's mouth and blow their stomach, grab whatever other organs you can think about and smash them against their bones. Write it ahead of time, so you can give them a clear image of the brutality (and honestly, gross) of their kill. Once you have described it once or twice, they'll be satisfied, and you can go back to saying "yeah, you attack with your tentacle and deal 1d6 points of damage".