Kiddie tax and the new tax law

I have a seven grandchildren who are all dependents on their parents' return, so they come under the Kiddie Tax rules. To put money away for their college, I gift highly-appreciated stock that I bought 30+ years ago. What is the maximum long-term capital gain that they can have and still pay no federal income tax. I think it's somewhere between $2500 and $5000, but I'd like an exact figure and itemization. For example, if their sole income is long-term capital gain (or qualifying dividends), do they get a standard deduction, or is there no standard deduction without earned income?