Agree with others about temperature, it does feel several degrees hotter there than every other park. Bring a change of clothes, as there are several fountain-style things they can get wet in (unlike the actual fountains, please don’t send your kid in there!) and cool down a bit.
That said, it is generally one of our best days there, with little kids. We always do Rafiki’s planet watch (petting area is fun, cute train ride to get there, couple of characters to meet with little or no line (Jiminy Cricket and Rafiki)), my kids love the aquarium and hippo areas, and we spend a ton of time walking around the trails (kangaroos, gorillas, tigers, etc.). Unlike other parks, it is where my 1.5 year old could run around a bit more. He also loved the dino area, we have had fabulous M&G with Pluto and Goofy there, and the dumbo-style ride there is his favorite.
As for choosing between the 2, Safari would win hands-down, no contest. I am not a huge fan of the FotLK, so we haven’t taken the kids there, btu Safari is a must-do for us. Early morning is great, as animals are more active and then you can just chill and walk afterwards until time for nap.