couple questions: would an 18 month old be able to ride with us? does the gem mining allow for you to actually mine for the gems or do you just purchase the bag? what is the best package to allow for all 4 of us to attend each activity?

Yes

They will allow an 18 month old to ride with you. But it is a full hour and there isn't a shorter option because the trail is a loop. So that age will likely either fall asleep (it's very relaxing ride) or get tired and irritable. We took a 2 1/2 year old, and she loved it but they were very surprised that she stayed awake the whole time. A five year old boy in our group started asking when it would be over after about 45 minutes.

Not sure about the 18 month old, liability and all. You purchase a bag for gems. There are plenty to choose from. Look at their website. You may find a discount.