Very cool projects! It is always great to pass on knowledge to our kids that seem to be getting lost these days. I am the same in that I am constantly teaching my kids about self reliance, sustainability, survival. Kudos to you for sharing that with their classes!
As for animals or crop, a lot of things depend on how much and type of land / climate you have available. For most people with smaller set ups, smaller animals such as rabbits, quail, pheasant and chickens make more sense and what I would suggest. Ideally those who want to live or be able to live a subsistence lifestyle, need to work to find ways to make their system self sustaining, so crop becomes important.
On the other end of things, if land is available that can support of self sustain larger animals then the downsides are reduced, but as mentioned there are considerations with large animals. As an example our Ranch and pasture lands on the Big Island are pretty self sustaining for our cattle that are true free range, no extra feed. Rain and water supply is abundant for us and ponds / reservoirs never run dry. Of course certain basic infrastructure needs to be set up, but it isn't overly burdensome especially for small personal situations. However once in place, it can be very self sustaining with normal maintenance. We also do chickens, ducks fish and there is no shortage of wild pigs, but keeping domestic pigs is great. Goats and sheep would also do well. It is nice to have a milk producing animal if you can support it. In addition we have garden / crop production so it goes into the cycle.
Bigger animals yield a lot of meat at one time, so you must have the means to slaughter, prep, preserve or keep the meat. Smaller animals are infinitely easier to deal with and rabbits are about the quickest and easiest to take from cage to table with great yield per input of animal. Also smaller animals such as rabbit, chicken, duck, quail are a one time meal so preservation isn't necessary. It depends if you want to do a lot of work at one time, or do you want to do lesser amounts of work every day for prepping your meat.
If you are limited in space there is much you can do as London outlines. But if you have the land you have options and diversity is a good thing. Pros and cons for everything and your own situation will dictate much of what you can or cannot do.