My history is a bit complicated, there's no real rhyme or reason to it, really.
Our kids were born in 2010 (the year of the iPad

) and 2011, for reference.
1. I got an original iPad 1 (16 GB, wifi) on clearance at 50% of the original price when the iPad 2 was released.
It was mainly used by the kids to watch videos (upgraded from a 32 GB iPod touch they were using before (my wife won the iPod touch in a raffle at work)). At the time we did not have a tv, so our toddlers would use the iPod touch and the iPad.
Skipped everything after that up until the iPad Air 1.
2. A few months after the release of the iPad Air 1, my wife needed one for work, so we got an iPad Air 1 128 GB Cellular model, and a keyboard case to go with it.
At that point, we had the original iPad and an iPad Air 1 in use.
3. At some point, an electronics chain here did a promo of 100 euros of the price of a base 16 GB wifi iPad Air, + 100 euros for trade-in of any tablet. So I traded in the original iPad 1 for a 16 GB iPad Air 1, for a total cost of 200 euros or so.
I wanted to get more storage, but since this was a kids iPad, and the 100 euro discount wasn't available on other models, we went with 16 GB and rolled with it. Got a cheap sturdy universal case for it.
4. Then, 6 months or so later, the same chain had a celebration for whatever reason, and they offered double trade-in value for any tablet (with purchase of a tablet more expensive than the trade-in value). By this time the iPad Air 2 was released. I checked the trade-in value and it was 175 euros X 2, which is more than I had originally paid for the 16 GB iPad Air 1, so I traded it in and got a 128 GB iPad Air 2 wifi (which was 50 euros of on top of that).
5. Last year we got a refurbished 128 GB iPad Air 2 Cellular, to replace my wife's iPad Air 1. The kids now use the iPad Air 1 and the wifi iPad Air 2, and my wife and I share the 4G iPad Air 2 and a Surface Pro 4 m3.
The iPad Air 2 is a great tablet, by the way, imho.
6. Future possible purchase: I'd still like to get an iPad Pro 12" (at least 128 GB + 4G), but prices are still too expensive, and waiting for a good refurb deal. We'll see how prices go.But any future iPad purchase will be to a 4GB RAM one, I learned that lesson.