We had a family BBQ today.
It's my mums birthday and next week mine and ds's. He will be 6 yo.
We arrived and food was almost ready so told ds to eat then we'd get toys out. He put his head in his arms and sulked but I reiterated to wait 2 minutes eat then yoys but my sis got out the toys. Not a problem. ds kept going down the alley and after 3 times of telling him to stay in the garden I said I'd take toys away if he did it again so he stopped.
Ate went to field and had a great time ds got stroppy after about 45 minuts as too hot so went back for pudding.
A little later I said I'd have to pop and feed friends cat and would go when brother dropped friend home and sisters boyfriend had to take his parents to a party.
I had a little tykes car in my boot to leave for my cousin to pick up so I asked ds to help me load presents in the car and we would get the car out.
Told again not to go down the alley to front of house (with car) and stay in garden bit (large garden) as the toys were sill on the floor. He went towards alley so I told him again quietly and he did it again. As I was loading the car I asked him to help and told him again not to take the car down the alley or near toys or he'd break them and I'd take the car away if he did. Anyway he did so I told him to get out of the car asked him to get some more presents as a distraction. He returned to the garden with me and began to cry about the car.
Before I could comfort him etc my mum asked what was wrong and I said I'd asked him to get out of the car for not listening.
She got really angry at me and told me I ruined it by upsetting him but I pointed out he chose to ignore me and he was upset as he couldn't get his own way.
Anyway basically I was told how awful I was and I simply said I did not think it was ok for him to be rolling over toys when they could break. She questioned how many times I had told him not too 
and I said 3 but she got really cross with me and told me the neighbours could hear now and stormed off into the house.
I left at that point as ds was crying and I felt uncomfortable and rang after feeding the cat and went back for the cake etc.
Nothing more was said.
AIBU in thinking she shouldn't have undermined me and that if ds is naughty he should have consequences where ever we are whatever the event?