Could it be argued when Prince Philip and James Lovelock had children although they were aware of climate change knowledge and global warming it wasn’t as clear and the earth’s temperate as increased as more recent decades, so having a lot of children then wouldn’t have been seen as environmentally harmful as it is known in more recent times?
Possibly, but they were still relatively young men when they started working in their fields of interest. Even so, expert or not, it isn't difficult to work out the consequences to the world and impact on resources after a few generations if every couple 'replaces themselves' twice over.
I don't know about Lovelock, but Philip never seemed too bothered in retrospect about the impact of his family on world resources - yes, once the children were here, there was no way of 'sending them back'; but going on to have the whole family live in luxurious opulence was no means obligatory. In fact, imagine how powerful the message could have been if they as the RF had taken a lead in making big sacrifices to save the planet.
At any rate, it's the easiest thing in the world to tell younger people that they mustn't do something that you personally happily benefited from. Thinking also of all the MPs, nearly all of whom had enjoyed free university education and thought nothing of it, who decided - some of them frothingly - that young people were completely unreasonable to expect it without ending up in a massive amount of debt. I can't remember any of those MPs offering in retrospect to make any contribution to repay what they had had.