First, ALWAYS put a time limit on the invite. That way, parents are very clear prior to the party when they need to pick their kids up.
For an 8 year old party, I would probably do a 3-hour party. Two hours seems to go by really quick but it depends on what you are doing.
Another thing that I always do (and have appreciated as a guest) is indicated if lunch or some other food will be served. People have many different lunch and dinner schedules so, sometimes it's hard to know whether your kid needs to be fed lunch before a party. My son went to a noon party one time and I kind of assumed there would be some food served (besides cake). There wasn't. No big deal, but I would have given him lunch prior to the party. Most parties we go to do serve the kids lunch (or dinner if it's in the evening). If it's a 2:00 p.m., then it's not necessary. But, bottom line, it's just always nice to know.