Supersmash, consider this -Federer was playing more sports than just tennis, and he didn't get into it competitively until much later.
By all means, at the young ages it's best to play multiple sports and build the ABCs -agility, balance, and coordination. Have them get outside as much as possible, run, play, do whatever they want to do. By the time they're 10-12, they'll have a solid athletic base. At that age, drop it to maybe two sports they enjoy (hopefully tennis), and try to get them out often. When they're about high school age, start focusing on tennis completely.
Starting at a young age isn't a big deal, as long as it's kept fun. Tournaments and rankings are a big no, however, until 12-14. Most people who start before tend to get very caught up in it...and burn out just as fast.
At the young ages, just focus on building athletic skills.