... I think, if it were me, I would wait at least a few days to be certain the kittens don't show up, and also watch her...if she begins to not call for them it shows that *she* is giving up and letting-them-go, then I would TNR her asap.
Here is my first TNR experience:
I began feeding Malibu's litter at about 4wks old, and in the course of the first week I eventually saw all four kittens. At about 6wks old, Mallie had *had it* with me paying so much attention to her nest and she moved them and I didn't see them for a month...until she brought them back one morning (!) to play in the flowers of my front porch. So, I began putting food out again and she showed up with the babies every morning. On the 4th morning, I put the food near the garage. On the 5th (a Saturday) I had the garage door lifted open about 6" and put the food in the middle of the floor. I watched for them to arrive, and they did. They didn't see the food in the usual place and when they finally noticed it was in the garage, all of them went in. I waited about a minute for them to all be concentrated on the food, and then I used my garage door clicker to close it the 6" and I caught them.
What I learned from this experience with feral cats and kittens is if I have a chance to catch kittens, I do it ASAP before I *lose* that opportunity. I hated the month Mallie had those kittens away and I didn't know if they'd lived or died. I eventually seperated the kittens from Mallie when socializing was going too slow in the garage. Kittens went to my bathroom and Mallie went to TNR and was returned. She did walk around and around our house calling for them for about a week and then she stopped. Mallie became my first Georgia feral that I tamed and she is now a very happy and confident housecat that sleeps with me every night. Her four kittens went through a local adoption center (PetsMart) to find good homes and I became a foster-person for that organization.
Best of luck with the kitties you've been feeding, that is wonderful they found someone to care for them. I am sorry to say, I don't think it sounds good that the Mamma is calling and none of the 7 kittens are appearing, but I will keep hoping.