OK, this might be tangential to what you are really asking, but...
If you use the default multiverse in DnD, think of it like this:
A summoned creature isn't actually there - only a temporary projection is there. In the same way that a character that projects to the Astral plane has a temporary existence there, but cannot be permanently slain there. (Unless the Silver Cord s severed.) An Astrally projecting character can even project onto another plane, interact with that plane etc, but cannot be permanently slain. In all cases, the character's soul returns to it's real, physical body. Summoning spells pull an Astral projection through the Astral plane and create a new form on the Prime Material plane. The same holds true for equipment. New equipment is formed on the plane, but the original equipment is still on the home plane.
So, when a summoned creature is killed, it's soul simply goes home and the formed body (and related equipment) dissipates. I would not allow a Dimensional Anchor to force the body and equipment to remain on a summoned creature.
Your players might not like this, but remember that it works both ways. If they project to the Astral plane and then go check out Hell by passing through a color pool, they can do so with relative impunity.
Of course, this is completely different if they Plane Shift there, or use Gate, or any other for of true dimensional travel instead of Astral Projection. For true dimensional travel, souls, bodies and items are truly moved to a different plane. Then, the stakes are real.
Now, if you don't use the default DnD multiverse, feel free to adjudicate it as you will. Just remember that you might put yourself in a position where your players start summoning beasties so they can dimensional anchor them and take their stuff.