T.F.
I am so sorry for your loss today. I think a bush/plant is a wonderful idea.
When we lost our first Cocker (14 yrs ago) , a friend gave us a pretty plaque for outdoors. I still have it out by the front door and we always think of Maggie when we see it. We also had her cremated and she is in a locked cedar box with her name engraved on top and sits on our mantle.
We also a rabbit and 2 other Cockers cremated and in their own cedar locked box on the mantle.
We just lost our 14.5 yr old American Cocker this past Dec. They are a huge part of the family. I still mess up and call Fluffy on occasion and when I do our other 2 dogs, (9 yr old English Cocker and 8 yr old Toy Poodle) come running. It is also sad when we are outside because Fluff LOVED our backyard. We've noticed the other 2 dogs sniffing around outside like they might find him.
It has been a big adjustment for our family and my daughter is 19. I think no matter the age, there is a bond with our pets and it hurts greatly when we lose one.
I am sorry for your loss and I hope your nieces will be adjust and like your idea.