Sound like a great plan!
My daughter goes to high school across the street from Grant Farm--I think I drove by it five times yesterday

. There are discussions about it being sold, so it's good you are coming to visit this year. It is closed on Mondays, admission is free, but parking costs $12. If your grandchildren are Civil War history buffs, you could stop in Whitehaven, a national historic site that is across the road from Grants Farm.
If you want to stay at a Drury and your plan is to visit the zoo, City Museum and Grant's Farm, I'd recommend Drury Inn Brentwood, hands down. The hotel is only a year or two old and centrally located to all those attractions.
Chesterfield is a wonderful area, but it's about 12 miles west of Brentwood/Clayton on a busy highway, especially during rush hour. You would literally be driving by the Brentwood Drury on your way to the zoo and City Museum. And Brentwood is only about 10 minutes from Grants Farm (take Hanley Road/Laclede Station south, right Gravois Road, right Grant Road).
Another stop I would recommend is Ted Drewes frozen custard. It's a St. Louis institution.
Have a great trip! Send me a PM if you have any questions about where to eat, park, how long it takes to get around, etc.