You reminded me that my 6yr old has asked to have his removed and learn how to ride like his older brother this summer. Whoa, it makes me tired thinking about it!
First of all, I'd like to say that I'm home alone with 4-6 kids on a regular basis and it shouldn't stop you from walking up the road to a park, going to the grocery store, or anything else really. The only thing I don't do without a second adult is my own doctor's appointments or dentist visits. But you know, sometimes I don't even have that choice if hubby is deployed. He works two jobs and also subs in for a contractor when he's home, so he's not home often. We sure enjoy him when he is, but it can't restrict my life that much or I'd never get anything done!
I think that if you tried it a couple of times you'd find out that it's not that bad after all. The kiddies stay in the playground area of the park with clearly defined boundaries and an understanding that _leaving_ those borders means dreaded time-outs! Then you take the one you're working with in circles around said play area on the bike with half an eye on the rest of the brood. The hardest time I've had with that is when I have little tiny babies, infants, or just toddling babies. Those guys have to ride on my chest in snugglies or on my back in a toddler pack if I need both hands. I have taken as many as 12 kids to the zoo before; I don't go on crowded weekends - I take my homeschool and homeschooling friends' kids on school days for field trips. We have buddies, head counts, and stay together as a group. With four 1yr olds and at least one under a year we do end up with a wagon load of littles. I've found out that 6yr olds are at a great age to be happy about pushing umbrella strollers all over the hilly zoo, so sometimes that's an option if I have more than 4 kindergarten aged kids. (Having a huge fenced in yard with padlocked gates helps too - I can cook dinner with a ton of kids in the yard and know that the dogs would bark if a stranger walked up and the kiddies are CONTAINED...)
That's how my 9yr old learned - the 3yr old was swinging and sliding, the 3mos old was in the snugli, and I ran along beside as he (then 6) went 'round and 'round. He didn't get comfortable on the bike that summer. Hubby was home the next summer and we took him to the parking lot behind a church/school up the road. That's when he finally got comfortable enough to ride up and down the alley behind our house (it's quite hilly, but no cars except the two who use driveways back there). He's been on his bike constantly ever since. He had some powerful motivation - he wanted one of those Razor scooters SO bad that year and we told him that he couldn't have anything else with wheels until he'd learned to ride his bike sans trainers...