I'm thinking of making a LEGO table but my two children both use different LEGO sizes. My oldest uses normal LEGO and my youngest uses DUPLO. I'm wondering if there is a base plate that both types of LEGO bricks will work on?
A regular LEGO base plate will work with DUPLO bricks. As demonstated in this question, the two systems are compatible in a number of ways. Here's an example of System base plates with DUPLO bricks attached:
It should be noted that not all DUPLO bricks will be able to connect to a System baseplate. Some examples of incompatibility are round bricks and some of the minifigs and animals. In order for these to connect you'll need a DUPLO brick or baseplate on top of the System baseplate.
What you need is a Nilo brick mat. It's a rubbery mat that has standard (small) lego studs on one side, and dulpo on the other. You can buy them on Amazon, Ebay or straight from Nilo. http://www.nilotoys.com/acc.htm
Regular LEGO baseplates can accommodate both LEGO and DUPLO. DUPLO obviously won't stick to regular baseplates as securely as on a DUPLO plate because the studs are much smaller, but they are compatible.
There are some pre-made LEGO tables that already have baseplates (or third-party LEGO compatible baseplates) incorporated in the top surface, or you can make your own table and cover the top with baseplates.
LEGO sells baseplates separately on their website, and they are also available at stores like Toys'R'Us. I have seen the pre-made tables at Toy'R'Us as well, and they are also available on eBay.