Explaining pet death to young children?

Cheyenne • Wife, mother to Jordan, Farica & Ciara. Native American, enrolled Chehalis, ASPCA, love dogs, work in Accounting, family over everything, USA

We have three great dogs who are 12, 10 & 8 years old. Our children are 7, 4 & 2. We’ve made the difficult decision to have our oldest dog put to sleep this coming week because she’s suffering and her quality of life isn’t there. Our two younger children are especially attached to all of our dogs. How should we go about explaining this situation to them?