Advice needed

So my nephew’s seventh birthday is coming up this Saturday and we are all supposed to go to the zoo it’s not the animals that I’m worried about it’s the heat this is my first pregnancy and it’s supposed to be 90+ degree weather with out factoring in the humidity. My husband does not want us to go as he is extremely paranoid about heat and my well-being and the baby safety. I don’t want to disappoint my nephew and I also don’t wanna hear it from her mother Who does not know that I am pregnant and that we plan on telling them Mother’s Day weekend. Any thoughts or advice on how to handle the situation would be greatly appreciated.