It's a tough situation OP and I understand your concern.
For my part, DH and his ex jointly agreed that DSD would isolate with her DM during lockdown as she has asthma.
We are missing her like crazy (despite lots of calls/FaceTime etc) but the priority was her health and though we are strictly following govt advice DH decided that the additional risk of mixing households couldn't be justified, so we are just going to sit it out and look forward to spending loads of time with her when we get through this.
I only mention this to demonstrate that lots of parents are making difficult choices that prioritise their children - unfortunately your Ex isn't one of them.
So what to do....
The reality is that the risk is small, especially if your children have no underlying medical conditions.
That said, as you rightly point out, he is increasing the risk and obviously sees no issue in doing so. As such it's fair to assume he will do it again (and again).
So for my part I'd put the onus back on him.
Don't say he can't see the children, say in light of his visit to his girlfriend he needs to isolate for 7 days after which (assuming her doesn't become ill) he can have the kids.
If he refuses to isolate for 7 days then it's really become his choice not to see the children rather than your decision to withhold contact.
Make sure you put this in writing (email/text) so if it come up further down the line you have this documented.
He he does isolate then dies another trip to his GF then the process starts again.
Obviously there is still the chance he'll agree to isolate but not do it and lie and equally not tell you about any visits to his GF in the future.
There's nothing you can do about this, but at least you've hopefully made him think about these unnecessary risks and that there are consequences to them.