In this particular situation I'm sure they knew that getting tear gas or pepper spray was a possibility. It shouldn't be that way but it is. However, I hold the people who attacked the child with pepper spray entirely responsible for the attack. It's like the situation where a woman is walking alone at night and is raped. 100% of the responsibility for the attack is on the rapist, who chose to commit the crime. The woman is not to blame for what happened to her. At the same time, most women would probably choose to avoid being in that situation if possible, because we recognize the gulf between what should be the case (everyone can walk freely) and what is (women are at risk of violence). On that basis, the breeders shouldn't have brought the child, but they are not to blame for what happened.
More generally, I don't agree with bringing children to protests because young children are just mimicking adults, so it is using a child to further your own agenda. I feel that way whenever I see children participating in forced-birther protests, which I very much disagree with, so it would be hypocritical if I didn't see it the same when when it's a cause I agree with, like BLM.
They don't need to be adults before they can have a say (after all, they live in the society even if they cannot vote), but until they are requesting to go to protests on their own, they are just a prop for the adults. And I say that as someone who was the kind of child who was politically active, but by my own choosing.