Can anyone tell me whether you can mount a child car seat restraint/anchor point into a beetle? If so, where is the best position? thanks

Yes. I had anchor points installed on both sides by an approved installer. All legal.

I have mounted them in a couple of my Bugs over the last few years.
I just used the ones you buy from Target with the big flat washer. I replaced the washer with a larger steel plate with a hole in the middle.
I drilled just below the top of the firewall where it is folded over and welded to the bottom of the rear window panel/rear vent /engine lid hinge panel.
Thought this would be about the most structural and accessible area back there, easy enough to get a spanner on the nut behind the fan housing.
I have also seen them mounted through the bottom of the rear luggage area floor but access is a slight issue to tighten the bolt/nut.
Given the safety issue though you`d maybe better ask advice from a professional before undertaking this work.
Cheers
Graham.