Does anyone have any good advice on minimizing altitude sickness in children? I've read about keeping hydrated, eating carbs, etc., but wanted to see what others here think.

Last year, we did a day hike to a ranger lookout post at just over 9000 feet. Where we live is about 3200 feet. Neither I nor my wife felt any effects of the altitude (aside from the need to catch our breath more often). On the way back down, our son (almost 8) complained about a headache and got sick when we arrived back at the car. After giving it some thought, both me and my wife kicked ourselves for not being more attentive to altitude sickness before making the hike. We both grew up in higher elevations than where we now live, and I just don't think we feel it as much. (We felt really bad for letting it happen though....)

I think the main problem was doing this trip as a day hike and going up and down too fast. Does anyone have advice that would allow us to make this trip in a day and minimize the risk of altitude sickness for our son, or should we just abandon the idea for a day hike? Can we build up to it earlier with graduated hikes to higher elevations, or does it even make a difference?

I should clarify, the trailhead starts at just under 7000 feet and it's 3 miles to the top (approx 2500 feet elevation gain on the hike). It's only an hour and a half to the trailhead, so it's still a pretty significant altitude change overall.