Wee grandson wants to do some camping. We bought a cheap pop-up and the internal condensation is horrendous!

How expensive does one have to go to overcome this problem?

Any twin skin should do

+1

However, atmospheric conditions haven’t been helping recently. I bivvied out on Sunday night and everything was soaked due to the dew/mist.

And some double skin tents can still be bad.
Make sure it has good venting (that you leave open) and leave doors slightly unzipped unless it is horizontal rain or midge-fest.
They do not have to be expensive – most Vango’s are OK for venting.

Our cheap Coleman had a double skin and was pretty condensation free. Same with the little Eurohike one I occasionally use. I remember one camping trip where I got soaked putting mine up but my mate got soaked in the morning when he sat up inside his pop up tent.

If the air is damp or raining, and there is little wind, then you will get breath condensation. At least with two skins, it collects and runs down the inside of the outer skin, so the inner tent stays reasonable dry and comfy.