My older girls were 3 1/2 when the babies were born. Even though I was fortunate that they had a build in playmate, it was still a big adjustment for them, and it was very challenging to keep them happy and occupied during the first few months when I was constantly busy feeding or taking care of the babies. I think this was actually the toughest part of having a second set of twins. The good news is that it gets much easier as the babies grow, their needs become less all consuming, and they become more interactive. The babies adore the big girls and the the big girls are a huge help with entertaining the babies - they even routinely go and play with them in the mornings while they are still in their cribs so I can get a few extra minutes of sleep! Here are some things that helped me through those early months:
Get your older children involved in taking care of the babies. What form this will take will obviously depend on the age of your kids. For us, with 3 1/2 year olds, this meant sending them to run and get a diaper or burp cloth and having them make faces or sing to keep one baby amused while I was busy with the other. With close supervision, they were also able to hold and help feed the babies. On top of making the big girls feel useful and encouraging sibling bonding, they were actually surprisingly helpful.
Get some small activities for your older children to do while you are feeding the babies. I would feed the babies in the living room and try to set the big girls up at the coffee table with a coloring book, stickers, craft etc. If I was lucky, this would keep them occupied until the babies were done eating.
Set aside some protected time each day to spend with your older children and stick to it. This way your older children will know that, no matter how crazy the day gets, they will have some "mommy" (or "daddy") time with you. I made it a priority to read bedtime stories to my older daughters every night, even if it meant that the babies fussed for a few minutes while waiting to eat, and I know that it was a time that all 3 of us looked forward to each day.
Find some children's TV programs/movies that you are OK with. I did not let my older girls watch much TV at all when they were little, to the point where my mother-in-law found it borderline abusive and would try to sneak them cartoons whenever she had an opportunity. However, once the babies were born, it was invaluable to have them occupied quietly for a few minutes so that I could take care of the babies or take a much needed catnap myself. Although I still limit their screen time, there are actually some surprisingly good and educational shows out there.