Hi Robyn, I agree with all the moms who said kids don't have the dexterity to really do thorough job on until their around 7 or 8, however I do want to add that it's also a very good idea for parents to use a power brush. You and the child can really benefit from one of these, for example; 1)most of them have a timer, this gives us some time guidance and also teaches kids from and early age to brush longer then 30 seconds, 2)using a power brush does a much better job cleaning their teeth then a regular brush, it's electric so it does all the work for you which assures more plaque removal, 3)this one is my favorite...for a young child who hasn't been to the dentist yet it can give them some familiarity, what I mean by that it, the cleaning tools the hygienist uses such as "Mr bumpy" aka their hand pieces is not a foreign feel in their mouth. The power brush has some of the same sensations in their mouth...bumpy, noisy, etc. This helps so they don't freak out when the pull that out. You can warn them ahead of time letting them know that their going to use something similar to clean their teeth. Also, this can help should they need a filling - same idea. I highly recommend an Oral B rechargeable brush, you can pick one up for about $20 (its not fancy but it gets the job done), you could also get and Oral B kids battery brush but I find that it could be like a novelty, once the battery dies we discard it and forget about it. But what it comes down to is, what ever motivates your child!
As for the wiping thing, my son is six and has been doing the "wipe" thing for about a year now - I do however check every now and then just to make sure he's got it down. Learning to have good hygienic habits is essential from and early age.
Good luck on both topics!!