I tend to have a "setting in" period when a child first starts with me. We may have agreed verbally on the drop-off/collection times, but this two weeks can show up if we've got it right. If a parent is always collecting earlier/later (they might have underestimated the time it would take to drive across town?)), then I can suggest putting a different time in the contract.
I do have it written in the contract that late collection, unless previously agreed (which would be at the standard rate), will be something like £5.00 for up to half an hour.
However, there is a world of difference between a parent who is habitually late because they've popped into the supermarket, chatted late to a friend etc., and a parent who has had some kind of work emergency, or an occasional unavoidable delay.
I do have parents who work in professions where there may be an work emergency, and it would be nigh on impossible to ring me to say until much later.
If you have parents mucking you about, put it in a contract and charge them, or even suggest an early revision to the contract. If you have parents who do not deliberately cause you grief, I would still write it in the contract but you can choose not to charge the extra money. I would still make them pay for the extra time though (at the usual rate).
Being early to drop-off - again, if it is an occasional indiscretion, I would charge the usual rate, make a joke about charging them double time so they know that they're a bit early, but let it go. If they are always late, I would change the contract.
Phew, that was a long one, and I am supposed to be getting ready to go out for dinner!