I am very lucky with Jack(14 m) because he is not a very mouthy baby and never has been but Drake was! I would try to teach them not to put it in their mouth first by stopping them 2 or 3 times and then removing the activity. If they just aren't getting it(most babies won't) then here are some ideas that I have used and heard.
-Nap Time! Do school activities with the older child during the babies nap time.
-Kitchen Table. It's high enough that babies can't reach and you could strap them into their high chair with a safe activity or snack.
-Put the older child in a play pen with an activity so that the baby can't get to them(Caden LOVED this).
-We put Jack in a hook on chair in the school room so that he can work along side us without getting to the choke and breakables:)
Hope this helps:)
(In this picture Jack is putting dimes in a jar through the slit on the top. He loved and tried to put a dime in his mouth one time. I stopped him and said no eating and that was it. He loves activities like this!)