Honestly the main reason I took DS on the march was because it was easier than sourcing childcare, nothing to do with wanting him to experience it actually. I still disagree that it's exploitative. If it was likely to be a dangerous protest or it was too far to walk or something, that would be different, and it would be unfair to involve children. But when it's physically neutral to them, and the concept is over their head, then it is the adults' decision.
I do think children's consent is important. It's hugely important, actually. I just think that it applies to smaller individual interactions, like whether they want to be tickled, not something as broad as what their parents are interested in or whether they get taken on a march. Otherwise you'd be involving very small children in decisions like whether to go on holiday or move house which is just nonsense - they are too young to be part of those kinds of decisions because they don't understand all of the factors involved. So the adults around them have to make the choices, while, of course, avoiding unnecessary distress to children, and the children get to watch and observe this tapestry of life. That is, after all, how they learn to be adults.
You can't spend your entire life avoiding anything that your children might not understand. That's not practical, it's also isolating to women, who tend to be the main child carers, and also how the hell will they learn to understand things if they are never exposed to them?