I agree with thatbags re cleaning. I always cleaned my children's teeth, just letting them have a go right at the end. They still have good teeth - I don't think my 33 year old son has had any extractions or fillings.
I try to brush my grandchildren's teeth when they stay over but, despite trying to make it fun, it's an uphill struggle as they are used to doing it themselves when at home. In my grandson's case (2 yrs 6 mths) , this consists of him wetting the brush, putting it in his mouth, sucking the water off it and spitting out water- not ideal.
I think fruit juice should be diluted, especially as it is what they drinks most of the time, but nobody else in the family does this. As a child, I mainly drank water, and a treat would be when we bought a couple of bottles of Corona when the lorry came round. I very rarely buy my grandchildren sweets.