Dividends are investment income and subject to the "Net Investment tax", so there is a threshold at which the tax on dividends changes. But that's academic. Form 8615 is required and the parents income on line 6 is required.
It's highly unlikely that the bottom line will change. That's why it's OK to use estimates of the parent's income. I wouldn't even bother to double check. If you're concerned, then file an extension for the kids too and file their returns in Oct.
AMT does not come into play.
PS- I had to do form 8615, with capital gains, before there was tax software. I did invent a short cut. Today, TT does it for you and/or your accountant.
