We were in the sad situation of losing our cat suddenly a few weeks ago. My youngest is the same age as yours. I found her (the cat) on the road when I came home from work, she had been hit minutes before that. We were able to bring her home and let the girls say good bye and the next day we buried her in the garden. Like you, we live in a rural area with plenty of room so we could chose somewhere nice for her under her favourite tree.
As time has passed we all still miss her so much, the girls put flowers on her grave and I've noticed the little one sometimes goes and sits under the tree and has a wee chat. We don't labour the point or anything but if the girls want to talk, we listen. The upsets are getting further apart now but every now and again the little one gets a bit tearful and needs to talk.
We have two dogs who have been a great comfort and we know that one day we will get another cat but not just now. I feel that when the time is right, we'll know.
Sorry for your loss, it's so hard losing a loved pet.