Age- 12 to 14 weeks, I'd guess. I hope you are going back to the pond to search for other litter mates! There could have been one or two or three more hiding somewhere nearby. If mom was dumped, they would likely stick by her. I'm assuming they are friendly which tells us they were handled by people already... Please do a few searches. You could put one kitten in a carrier and put it in some bushes (or wherever it seems likely a cat would hide with kittens, or kittens without mom). Kitten in carrier would hopefully start to cry in a half hour or less- attracting litter mates over. You'd have to situate yourself somewhere where you could see the carrier but far enough away to allow any littermates to feel safe to go to the cries- you wouldn't want them to be able to see you, in other words. After the kitten starts to cry, give it a good hour for any possible littermates to go to him.
Just saying what I would do!! Good luck with the ones you have, they look adorable! Let us know what the vet says about their health!