The best math & science toy especially for that young age is probably an interactive adult and going out into nature! :-) But I'm assuming you were thinking something more along the lines of things you can buy for said two and four year old.... (one note, a lot of these things might say for older children, but that's often because of small pieces - if they're going to be used with supervision and with interaction, then they're good for younger!)
On the math side...
* Tangrams are fun for building spatial sense - http://www.learningresources.com/product/139345.do
* Building patterns is a good skill, can certainly do this with household objects as well - http://www.learningresources.com/product/beads+and+pattern+card+set.do
* Basic 'move your piece along the number line' games are really good. Chutes and Ladders works, although for younger kids, you often have to create your own game on paper with fewer numbers. http://www.parentingscience.com/preschool-math-games.html Somewhat surprisingly, I don't know of any commercial versions, despite strong research backing... http://www.psy.cmu.edu/~siegler/sieg-ram09.pdf
* Puzzles are great for building spatial skills, and if the little girls happen to want pink, princess stuff, you can still sneak in puzzles! http://news.uchicago.edu/article/2012/02/15/puzzle-play-helps-boost-learning-important-math-related-skills
* Legos or other building blocks also build spatial skills - http://www.huffingtonpost.com/roberta-michnick-golinkoff/a-chip-off-the-old-block-_b_4640310.html
On the science side...
* Different kits to experiment with what happens in different situations are good. http://www.timberdoodle.com/Primary_Science_Set_by_Learning_Resources_p/047-215.htm
* Bubbles are always fun!
* Lots of experiments can be done with household items - http://www.parentingscience.com/preschool-science-activities.html
* Tickets/passes to local science museums or zoos - I'd love to go to this Little Makers program! http://nysci.org/little-makers/
On the tech side...
* I'm still enamored with Robot Turtles - http://www.thinkfun.com/robotturtles/
* LittleBits are cool - would definitely need supervision and support though for youngun's - http://littlebits.cc/kits/base-kit
* ScratchJr is now available - http://www.scratchjr.org/