Two days is not enough!
My 5 year old and I came to Paris and London in March 2013 and it was the coldest March in 100 years in London - complete with snow.
Where else are you going? Bring warm, waterproof layers and if you need more, hit H&M, Monoprix or other cheap and cheerful shop for hats and gloves. We did that in London and got a cute hat which reminds us of that trip. There will not be much selection as it's supposed to be the end of the season.
I noticed that everyone - men and women - wear scarves even when it's only a temperate. I went out yesterday without one and felt naked!