My first thought was "which tweet?" as there have been so many of them.
But as someone living in the US and following what Trump tweets, I would take anything he tweets with a huge grain of sand (maybe even a whole beach or two).
I don't know what the political/social situation is in Iran (other than what I see in the news here and that there is possibly a general feeling of unrest), so I can't attest to how safe you and your family would be during your trip. That is up to you to decide.
However I would say that no tweet of Trump's is going to change what the current situation in Iran is
In general (in my personal opinion - others may disagree with me) what Trump tweets is rhetoric designed to appeal to his voter base and not to cause change in other countries.
As for your second question about the US visa. In general just visiting Iran will not be cause for a visa refusal. It may cause more scrutiny of your application (but I am not a State Dept employee - so I don't really know). I just googled it and there are many people who visit Iran and then visit the US without an issue.
The only issue is if there is some obvious nefarious patterns in your travels that you need to explain (I was once a tourist in Syria - which was a great place to visit before the war, but going there now would be a huge red flag) , or if you are a national of the country de jour that is banned by the current administration from coming to the US (and even that seems to change daily based on court injunctions etc)