Son #1 had gastroenteritis when was under a year old. He did end up hospitalised and on IV fluids because he lost weight due to diarrhea.
After they were satisfied the infection was under control, they sent him home on Pedialyte and liquid Jello, then onto a special formula (can't remember the name now, but the bloody thing was like $5 for a very small can equivalent to two bottles).
After that, I've always been careful with little ones and diarrhea. If it appears to last more than a few hours (in other words, more than just a quirk), I always try to get them to take Pedialyte.