The areas to the front of World Showcase will start to be taped off hours before Illuminations. And by taped off, I mean tape on the walkways. You can sit in between the tape and the lagoon as early as you want. The walkway will still be open for others to, well, walk.
In the back of World Showcase, towards Germany through Italy, there won't be tape but people will start to stake out spots about an hour early. Personally, getting the family some food and drinks in the back of the Showcase, then all sitting on the ground and waiting is a good option. If you go around Germany, there are close bathrooms and drink options for later. After the show, you can make a slow exit to the front, stopping for bathroom breaks (Norway is excellent since their bathrooms are not in the front of the pavilion) and the again, slowly walking to the exit.
For a snack after Illuminations, you can take a zipper bag and get some egg rolls from China and munch on those on the way out. You need to purchase these before the show starts.