I would try to capture them as soon as possible to get the kittens socialized.. so they can be adopted out easier. I believe they are also old enough to be spayed at 12 weeks, but I'm not sure. Are they eating regular cat food? An excellent website that you might want to look at for help is http://www.alleycat.org for the Alley Cat Allies and there is much info on feral cats including humanely trapping them. Good luck! You are doing a great thing!