Child, ready for school in the morning.
"Do you have everything you need for school?" is asked.
The answer is usually "Yes."
Upon inspection, the kid is actually not.
This is normal. This is perfectly normal. Your brain isn't up to speed yet? Great, neither is theirs.
Do point out what they are missing (and why it is necessary if it's non-obvious). Try to avoid telling the kid that they are lazy, messy, forgetful, or anything that you want them to not be. Telling them that they are will convince them that they are, not make them work to change it. Telling them that it helps if they're more alert, and try to remember things more will help in the long run, and calling them on it every time is necessary.
(Except, of course, for cases where you've had to remind them about something they keep forgetting for more than a week or more than a month. If it's sufficiently non-critical (forgetting mittens/hat in Alaska is a Bad Thing; forgetting homework is not life-threatening), you may want to tell them that you're going to stop reminding them about it, and that they'll have to remember on their own. Then, of course, you must not remind them the next time, and let them go to school without whatever it was.)
Often, "I think you're forgetting something" is sufficient clue to have them look around and figure out for themselves what it is that they're missing without you having to tell them. When they're used to figuring it out for themselves from that cue, then asking them if they're all ready may cause them to do a self-check, and may get actual results.