I will tell them and show them, since I am time traveling I would bring a computer and plenty of videos how the future will play out and then make sure they put mechanisms into place to keep that from happening. All I would do is show them things like below.
1. Slaves, when you explain the likes of Jesse Jackson, Al Sharpton and the other race hustlers say, and how affirmative actions plays out. Make sure you get rid of Slavery NOW. I can see George Washington standing up during the constitutional convention and saying. "My fellow patriots, I have seen the future and there are two men called Jesse Jackson and Al Sharpton who should never be allowed to be born and we MUST send all the slaves back to their country of origin and muster all our forces after we achieve independence to rid the United States of the institution of slavery." Just showing them a video of these Charlatans would cause everyone of our forefathers to get rid of their slaves and send them all back.
2. The runaway spending on social programs, make sure they address that PLAINLY. Like the GOVERNMENT SHALL MAKE NO PROVISIONS FOR PEOPLE WHO ARE LAZY AND WILL NOT WORK.
3. DEFINE more explicitly the First Amendment when it comes to the establishment clause and the "so-called wall of separation".
4. Tighten up and reign in the power of the supreme court.
5. Get rid of the militia wording in the second amendment.
6. Show them what taxes produce in the size of the government. make sure they use explicit language to prevent taxes
7. Show them what a career politician looks like (Harry Reid, Ted Kennedy) that would scare them into creating language to prevent them.
I am sure I can think of others