How lovely to have her interested in science from such a young age! As a science teacher this always makes me happy hear.
Unfortunately all the decent whizz bang sets are for at least 8+ but depending on what you think you could always get a basic salters one?
There are a lot of crap ones on the market, but anything from the national geographic will be excellent. There is a good potato clock one included in the boots 3 for 2 offer.
Equally, the science museum has an online shop with decent kits.
Finally, the Richard Hammond ones look good, as he has a huge input into the contents and is passionate about science education.
There are lots of excellent ideas for at home science experiments on the Planet Science website too.
The classic make a volcano is good fun, all you need is bicarbonate of soda and some cheap vinegar. Adding some food dye and a little detergent makes it even more bubbly. You can also make an impressive lava lamp but getting a big cylinder, an empty clear bottle works. Half fill with cheap oil and top up with water. The night before make some ice cubes with food dye in them and when you out them in the cylinder you will see oil covered droplets float down until they get to the oil water interface when they will burst.
I can't wait for my baby to be old enough to do these with!
HTH.