I agree with the "rules" of restricting lav use to cabin class within reason.
Jane Doe seated in 12C on a domestic 2-cabin gets up and goes forward to the lav and is the only psgr in the aisles. She's told to use the aft facilities.
Correct, IMHO.

Janey Doe is 4 yrs old and has to go and there's a drink cart blocking the aft lavs and her parent takes her to the FC lav. Acceptable? Yes.

Switching around gender and age for the above scenarios doesn't change my opinion.

It's a toilet, for goodness sake (or WC, for my European friends). We all need one, regardless of class of service for this particular ticket purchase.
It's perfectly reasonable for a customer to go to the most accessible facility during the boarding process when the aisles are blocked with other boarding customers. Ditto during meal/bev service.

I once had a psgr refuse to deplane. The FA's could only tell me she wouldn't get off the plane. They had no explanation nor would she give them one.

I went onboard and asked her if there was anything I could help her with. (The a/c was needed for a quick turn- 30 mins. arrival to dept.- and we had wasted at least 20 mins. waiting for the inbound flt. to deplane.) They were both over 70 yrs. old by my estimate and her husband explained that she had been told she couldn't use the FC lav and that the drink cart was in the way in the back, so his wife waited and had an accident. It was a 45 min. delay accident requiring her baggage to be retrieved and for yours truly to assist in trying to repair what was left of her dignity. (It was a bad accident.)

To force anyone to explain their urgent need for a lav when there are people or equipment blocking the passage to "their" lav is close to inhumane. There are some who won't tell you to "step off or put on your rainboots".

Obviously, I'm one that will.