Hi Alice, can't speak to the Grill for 1 and 3 but we took my grandkids to Tiffins and they were fine, although I think they were 2 and 3 at the time. It wasn't busy when we went, we went in at opening and didn't have an ADR but now I think that's more necessary.
As far as any restaurant, I've taken ours wherever I want to go if the restaurant doesn't prohibit children. Same with my own when they were small - my youngest who is 28 this month dined happily at CG when she was 4 and onward. It's me that's sort of burned out on CG, not that it isn't good, I just want to try everything.
The only place I ever had to remove with a grand was a Sunday brunch in a very casual tavern - she went all wackadoodle on us, plus knocked over my orange juice onto the bench seat where I was sitting, screamed and generally caused a ruckus. Didn't see that before and haven't seen it since, but it can happen. I took her out so the rest could eat in some kind of peace and I was all wet anyway. Made me wary for the following dining events but they turned out okay.