We did this when ours were 6 and 9 and they both enjoyed it. If your 9 yr old is at all extroverted, they can engage the CM with questions. We got to release ladybugs and feed the fish and try a fresh cucumber. It was fun. Military and I believe AAA both offer discounts.
AAA is not a discount for AAA card holders, it's only available with the AAA Diamond Vacation card, which you get by booking a vacation package through AAA.
wdwinfo has a helpful list of the tours with available discounts.
We got day of, right after we got of living with the land, we had to wait 30 mins but was definitely worth itHow far in advance should one sign up for the tour? Specifically, is there usually availability day-of in late May?