I have multiple iPhones and iPads at home. So there are chargers for both lying around in different rooms. I also have some in our backpacks. This means that there are frequently times when I am near an iPhone charger or an iPad charger with the opposite device wanting to be charged.
I confidently use the chargers interchangeably with the devices. Charging the iPhone with your iPad charger is harmless. In fact it charges the phone faster!
I also often charge the iPad with the iPhone charger. There is a minor penalty in terms of speed of charging, but the impact is negligible because I usually charge overnight and I don’t even notice the difference.