How to tell kids about dog passing

I might have to put my 15 year old dog to sleep today, his kidneys are failing, he has severe pancreatitis, grade 3 heart murmur, bad arthritis, and a collapsing trachea. He isn’t eating and is BARELY drinking at all(we have to use a syringe to get him to drink so his kidneys wont fail) , and is throwing up. Also he was given pain medication but can’t hold it down. I don’t want him to keep suffering and being in pain so I’m asking the vet today if he can recheck his kidneys and pancreas. If it hasn’t improved then I’m going to put him to sleep.

Does anyone have any advice on how to explain this to a 5 and 3 year old? I tried hinting to it and explaining a bit on Friday when this all started but I’m not sure how to put it when we go to say goodbye