Partner is claiming I started a war with his parents because I would answer their phone call, he says I was being ignorant.

Back story my partner had promised me he would be home Saturday evening 4-10pm to watch our son as I had planned a evening out with my sister.
Few days before he now informed me he has to go clothes shopping for his best friends wedding on the Saturday. The shopping plan changed 3 times within the 24hrs I heard about it. This upset me and I asked him to find child care for our son. He agreed and asked his parents. Im not a huge fan of them watching him as they are low effort and not the most warm grandparents. But the arrangement was made that my partner was to deal with his parents.
I told my partner my plans for that evening. Leave at 4pm, arrive at 5pm. Eat and one game of bowling then movie at 7pm-9pm, be home by 10pm.
His parents called me, which they haven’t for months. My partner was to tell them all the details and take care of the arrangement. I honestly missed the 1st call as my ringer was off. And was on the other line for the second call. Which is when I texted my partner as to why they would be calling me. They told my partner that if I didn’t answer their phone call they wouldn’t pick up our son. And once I heard that I thought fck them. Do as I say or else doesn’t fly with me. And from then I decided I will never need their help again. And wont go out of my way as I have had to before for them to be part of me and my sons life.