Were it me, I'd try to catch and tame them in an enclosed area. IMO, if kittens are old enough to go on field trips with Mama, they are old enough to nab and tame/hand raise. When Mama takes them on field trips, they are usually at weaning age. I dislike taking them from the mother at such young ages, but if it comes down to catch them now to tame/s/n or miss the opportunity...I'll catch them. If all you wish to do is TNR, I'd just try to get them used to you and TNR them whenever you feel they are big enough for your vet to do them.
However, I'd wait until they were independent enough to be released on their own after the surgery, because the chances of Mama kitty being nearby at release, or kitten knowing how/where to go, are pretty slim. You also run the risk of missing the opportunity to catch them as they get older and maybe move on for different territories, though they usually stay near their birthplace unless something drives them off.
I have trapped a litter of kittens that were about 9-10wks old and they tamed nicely and were all adopted into homes through the PetsMart adoption program. I kept them in a bathroom and after they allowed me to handle them (cooked chicken is irresistible!) I would let them into the Master BR during the day, bathroom only at night. I never let the fosters mix/mingle with my own housecats.
Good luck and keep us posted as to what you're able to do,