what could they be?

[Herpin Man]

Are you sure they aren't simply neonate isopods?

[KayJoh]

After more looking online I am pretty sure that what I am seeing is a soil mite. They don't look like an Isopod. I did have some tiny little baby Isopods and these look totally different.
These are quite round with little legs more at the front.
As I said I sorted the Isopods out and put them in clean substrate but should I add them to my vivarium or is there a risk of still getting these little bugs in there from the Isopods.
If they did get into my vivarium are they harmful?

[Herpin Man]

They are more annoying than harmful. They compete with isopods for resources. My guess is that they will find their way into your vivarium regardless, if conditions are favorable.

[KayJoh]

I haven't seen anymore in the new little tub that I have set the Isopds up in so hoping I may have caught them but from what you have said I wont worry TOO much now. As long as they aren't harmful to the frogs when they go in there. I'm hoping to get my frogs in about four to five weeks time so that should give the isopods and springtails time to sort themselves out.
Thanks for your advice.