I recommend this book.
https://www.goodreads.com/book/show/24396881-the-goodbye-book

Tell them what’s happened but realize the discussions around this will be going on for months, or years. So don’t stress too much about making sure then fully understand everything now. Keep it simple and always leave the door open for questions.

I’m sorry OP.

I told the kids our dog was sick and I was taking her to the vet. I didn’t want to put them through a traumatic goodbye. I went to vet with wife and had her put to sleep. When we came home we told the kids the dog passed away at the vets office. I felt this was the least traumatic for them.

^ (jerky DH was not home to stay with kids and did no want to drive back from work in time. I drove back from work ...)