Consider making a pawprint or clipping some hair to have as a keepsake. Create some rituals after the euthanasia to bond with your family. Go to the library and check out children’s book dealing with pet loss. I also have written one called My Heart Remembers My Pet that is available as a pdf.
Be sure to be gentle with yourself too.As parents, in making sure our kids are fine, our own sorrows sometimes takes a back seat. Kids are transparent. Like sponges, their open hearts can handle honesty. Allow your heart to be too. Sharing your feelings with your child will bring additional comfort.