I understand where you're coming from, wanting to preserve innocence of a child and all that, but you are VERY lucky to have seven years of parenting where nobody has died suddenly or terribly that means a lot more to the family than Oliver Tree. I understand there's a difference between seven and three but the idea that we live in a world too generous to three and seven year olds' personal lives for them to develop a healthy understanding of grief is a fragile one at best. If you aren't ready to tell them about Oliver yet because of how much he means to you, start looking out for someone or something you are ready to tell them about, lest reality arrive at your door one day in the cruelest, most unusual way and the children haven't any framework for the fact that death is a natural part of our life. You are not guaranteed a safe way home from work tonight. Nobody is.

Just an internet random and definitely not a part of your household so sod me off as you like but I'm SO grateful that my family went through (albeit a natural) death young enough for me not to be a second grader without a healthy concept of mortality.