If this is an outdoor cat, its likely that she may be infected with one of the big viruses and may have passed it on to the kits, or that her kittens were born sickly. If this is the case, its common for her to eat them, bury them and/or abandon them. Does she see the vet regularly? Is she vaccinated? Do you have a problem with coyotes or raccoons where you're located? She could also have detected a predator and left them, especially as she has never had a litter before (and this should be her last!).
I would recommend having her combo-tested and spayed. If you're in the US and need help trapping her (I know its not uncommon for outside cats to haaaatttte going into a carrier, in which case it is safer to use a human, live trap and then transfer to a carrier) and getting ahold of free/low-cost spay services, send me a PM
It will be safer for her to not be having litter upon litter and its better for the community to not add to the already large population of unwanted kitties. I know she's n outside cat...one of the things we rescuers are starting to do for stray, feral, barn and outdoor cats is to trap them, spay or neuter them, and vaccinate/test when funds allow and simply let them back to the outside territory their used to and keep an eye on them (as you're obviously doing with this kitty!). This is called TNR, or trap, neuter, return.
If you need any help or advice, drop me a PM! Good luck!