Trump was talking about "modifying" Obamacare repeal to preserve these two Obamacare benefits. I'm not agin the idea, but Trump ought to do it this way.
1. Have Congress pass, and he sign, a simple one page bill repealing Obamacare root and branch. Just to make a point.
2. Promise to sign a preexisting conditions law and a separate 26 year old children law, should Congress get its act together and pass them some time in the future.
If Trump allows "modification" of Obamacare, the special interests come out of the woodwork, all bets are off, all sorts of "stuff" will get packed into the "modification". Better to kill the whole thing, and require Congress to pass new legislation from scratch to pass out any goodies to the voters. Make sure to record the names of Congresscritters proposing and voting for such laws.