I have ABSOLUTELY no idea why I am reading this as my DC are in ys 5 and 7! But it makes me feel really nostalgic!
If your school gives out lists of term dates etc in advance, mark everything down in your diary. If you say to a small child "do you need to take anything in to school today?" you will not get a sensible answer. If you say "ooh, its assembly next week, what are you doing for it?" you will be told that they need a bumble bee outfit for tomorrow (#askmehowiknowthat!)
Deffo agree re not chucking stuff out too quickly. We have had bits of uniform too disreputable for daily wear, but they become very acceptable when other stuff has gone missing, or eg for trips and sports days when a discarded sweatshirt may just never come back. My experience has been that most stuff does turn up after a while, but it makes life a whole lot easier if you have something to tide you over.
My ds is and always has been the worlds most easy going child (he is currently applying his own particular brand of chilled out-ness to revising for Y7 exams 
) and even he was horrendous in the first term of reception, tired, stroppy, argumentative with sibling etc, but fine at school. You need to cut them a huge amount of slack - at home it needs to be a clear and undemanding routine, lots of food, regular bed times (and early! my niece used to fall asleep at 6pm!) Remember the goal here is nothing to do with anything academic - the goal is for them to enjoy going in to school and participating in what is on offer. If they can do that, you really are going to be fine.