Putting aside the ChemE question for a minute…
I think it’s a parent’s role to be their child’s number one advisor, advocate and cheerleader. Give them the best advise you can, but listen to what they have to say (if not, they will not listen to what you have to say), and be supportive of their choice.
In the case of choosing a major, it’s a very personal choice, that likely will have long term impacts on one’s life. They should and need to have the final say in choosing the major/career, but an 18 year old can use all of the advise (and support) they can get.
If they want to take a risk, explain that risk as best as you can, then be supportive and help them succeed. Those that do great things in life, are the risk takers. 
If “Bill”, wants to drop out of Harvard to start some little software company, tell him the risk, push back, ask him to explain the business plan, but at the end be supportive of his efforts. It could work out in the end…