Hi. I feel odd answering this because I feel like my life is in disarray, but people often comment on how 'together' I look so I must at least give that outward appearance. I suffer from a messy house like you do with 2 toddler boys and a cluttery husband - I don't have a chance, but I do tend to look 'pulled together' at the school gate - as you mentioned.
The key is to keep it simple and to pay attention to the detail. I am a SAHM and basically wear a uniform every day. Summer: Nice jeans, clean unwrinkled tshirt in a classic colour (a bit of rayon or polyester mixed in with your cotton will go a long way toward keeping you unwrinkled), a pair of ballerina flats. Winter: add a cashmere or wool vneck sweater to the summer outfit and replace the ballerinas with good quality leather boots.
For work, dresses are a godsend. You don't have to worry about matching, except for shoes, and it's quick. Just one piece on and go.
Accessories are also key. Wear them!, bit keep it simple. I wear pearl earrings, a watch, a simple silver necklace and my wedding ring. I also always wear a belt. I can't tell you how much difference this will make in making you look finished. A good quality leather belt will finish your look. If it has belt loops, belt it!
Quality is key. Buy the most expensive basic items you can afford in classic colors. Especially shoes. I find it so tragic when I see someone in a great outfit and they are wearing scratched up, old, cheap shoes. The right shoes can make a cheap outfit look expensive and sadly the opposite is also true.
If you want to buy something that is trendy, go for it, but don't spend a lot of money. Buy a cheap version, love it for the season it's 'in' and then get rid of it.
Fit is also key. Your clothes need to fit you right. No muffin tops or baggy tshirts. If you're not perfectly shaped (as most of us aren't) buy clothes that fit the good bits and skim the bad. Take a look at how you look in clothes when seated, not just when standing in front of the mirror. I started doing this and was amazed that I didn't look near as good in some of my clothes as I thought I did. Some went to the Good Will immediately.
You've had lots of good advice on hair and make up, so I'll stop there. I did want to thank you for asking this question though. I've got some great ideas for keeping my house organized. I appreciate it and I wish you all the best.