Up cycle fun tunnel

My son works for a pump co. He said he had a water tank that was unusable did I want it? Yes of course I want it.  Like always I figured I would make a raised bed.  When I saw how big it was I didn't know what to do with it, because it's so big I would never be able to reach the center.  My daughter was with me, and suggested I cut in a door. I instantly thought keyhole garden, I have always wanted to build one of those.  The next day I'm researching keyhole gardens, figuring out all the details. My daughter said she wishes we had 2 because she thinks it would be great to make entertainment for the dogs.  We have 3,  6 month old German shepherd mix puppies.  Because they got quite sick a few weeks ago we took out the giant oleander. Which left a bare and dusty yard.  They love to play in the kiddy pool, but we haven't been filling that for them Incase that's what made them sick.  They got fixed 6/10//22. So between trying to make sure they don't get sick, don't open there stitches, and no time to extend the fence, well it's not a very dog friendly yard.
My daughter has been watering, thinking even green weeds are better than dust.  Lucky the trees on the side of the yard provide shade somewhere in the yard all day. I will give up the keyhole garden, there are lots of ways to make that, besides the fact I need another garden like I need a hole in my head.  Anyway we will cut it so it's short enough to jump on top of, probably 3'. Cut out 3 openings in the bottom, so they can go in and out. Drill lots of holes around the side for air flow to try to keep it for getting to hot. I think I will put some shade cloth over it.  It seems like a great doggy jungle gym. Play on top, or under, out of the sun, or rain. Nap, or play, it just seems like it will be a good place for them.  If for some reason it doesn't work out, I can still repurpose it into a keyhole garden.

I am sorry the solution was to cut the tree down, instead of something else, fencing etc.
I do not understand that line of thinking.

Oleander is extremely poisonous. The leaf, bark roots , flowers, everything. I have never liked that tree, and never understand why someone would plant it in there yard.  It did provide shade on the west side of our house, so we tolerated it.   It wouldn't do any good to fence it. One leaf can kill a dog.  There really was no safe way to handle the situation.  I do plan to plant a new tree in the same space. I will have to fence it, because the puppies will destroy it for sure.  Before you get to upset I planted 7 trees, one clumping bamboo, and have another clumping bamboo to plant this year. Not to mention I really want a couple of avocado trees.  We are planters, not removers, but sometimes you just have to do what is safe for the pets you love.