In terms of being prepared - we often get a few lovely Sunday mornings in September, warm and dry - which are a great time to get started with no special equipment. As the weather changes, there are a few items which start to be useful. We don’t buy into “toughening up” u6s and are very sensitive to keeping the little ones protected (shortening sessions or cancelling if weather dictates) but it’s worthwhile having…

Gloves - Probably our #1 kit item is some gloves. Hands get cold quickest and many don’t like muddy hands!

Best - however expensive at about ~£10 if you shop around - optimum multi-x glove - The mini size is the smallest I’ve seen and is great for tiny hands. Optimum Sport
Value - Decathlon - if you happen to be visiting and can track them down. Decathlon

Boots - hand-me-down football/rugby boots are perfect. As the ground gets wetter it gets muddy/wet and flat soled trainers are a bit too slippy and/or get saturated. The club has a hand-me-down “boot shop” ask if you need more information.
Layers - John has posted a great note about tight base-layers on the minis facebook page. The wind is the main thing to worry about. It’s better to wear too much (and take it off) than to get cold. We find the little ones can’t get warm again if they get cold. You’ll end up carrying most of the layers after they’ve got going - but without them, you’ll just be carrying your child back to the car.
Hats/scarves - can also be good.
A water bottle and maybe a snack for half way through. If you find your player starts fast and then doesn’t make it to the end of the session, they are probably hungry as little ones burn their energy stores very quickly.
On cold days why not try having a thermos with a warm drink (mmm hot ribena) that they can have a sip of?

Please remember to name tag all equipment so it can find it’s way back to you when it gets lost.