Worried about death

Charlotte has been getting really upset about the possibility of me dying lately. She keeps mentioning death and has got upset almost every day for the past week (usually when shes going to sleep or waking up) about how if I die she won't have a Mummy. Its so sad, she gets so upset and shes even got me going a couple of times!

My grandfather died about 4 months months ago but she wasn't very close to him and I actually doubt if she even remembers him. I don't think she saw me upset at the time. Also our cat (who was staying with my in laws) died really suddenly about 2 months ago so we have had to explain death to her a bit quite recently.

I have just been saying to her that I plan on living for a very long time and hopefully won't die anytime soon. Have mentioned a few times that even if I did die I would still love her very much etc Have also talked about how old I hope to get eg I want to live for so long that I will be older than great gran and she will be old like Grammy and she'll'll have children of her own and maybe they'll even have children and we'll all be driving rockets... to try and get off the subject and make her feel better.

Has anyone elses children been through this stage? How long did it last and any more tips on how to make her feel better??? I remember going through a similar thing when I was little but I didn't think I was so young- I thought I was closer to 6 or 7.