School supplies should be under $25. I've been doing pretty good at catching the sales.
Clothes will be a little higher this year as DS needs new sneakers and a good quality backpack. This year, I'm getting LL Bean's $60 one because I don't want to have to buy another one 1/2 way through the school year. I would estimate no more than $200 for clothes, shoes and a backpack.
I have to tell you this was the best thing I ever did for the kids' supplies! I bit the bullet two years ago and bought them each a rolling backpack from ll bean, the total cost was like 169 dollars

but I was so sick of buying them a $20 backpack from target and having to duct tape it back together after three months and then buy them new ones by winter break.
They will be going on their third year of using the same LL bean backpacks and they are in perfect condition still. Seriously, I wouldn't have believed it, but I swear to god they can't kill these backpacks. So, I look at it like instead of buying 2-$20 backpacks per year, per kid, I've actually saved money by buying the one $80 backpack for each of them three years ago, with no end in sight in terms of the backpacks wearing out.
The girls and I are going to go through all their clothes today and see which winter clothes still fit, which summer clothes are suitable for school, and make a list from there for what they need, and bring the clothes that no longer fit to goodwill.
I've already bought them each some shoes that happened to be on sale, but for the bulk of it we're going to the outlet mall here tomorrow to take advantage of the tax-free holiday our state is running. I basically give them a copy of their lists (they're 9 and 10), and I have veto power, but I let them pick (they know the school dress policy).
I'm still anticipating being totally exhausted by the end of tomorrow, and several hundred dollars cheaper. I usually try for, eh, 5 new pants/kid, 5 new skorts/kid, 5 new longsleeves/kid, 5 new shortsleeves/kid, a new pair of sneakers, a new pair of boots, and a new pair of sandals (we live in the south, it stays warm here until november). Oh, and 3 or 4 sweaters. Their jackets still fit from last year. And probably a few other pairs of shoes here and there if they're a good deal.
I'd say we'll spend more than $200 but less than $500 for two kids for 6 months worth of clothing.