I seriously cannot think of anything the 14 year old and the 1 month old baby would enjoy this is a common gift...lol. Of course it would have to be something the whole family could enjoy.
If you can spend a few hundred dollars then I would suggest a larger screen TV and a selection of appropriate movies. Sort of like a home theater thing. Maybe they can open some boxes of popcorn and have a scavenger hunt to where the TV is set up in an out of the way place before putting it up in the final place it will be.
Then perhaps a years membership to the YMCA if the facility has a pool, game room, sports complex, and gymnasium. Something where the older child can go do the games and activities for the older kids while the younger kids go to the babysitting area to play on the massive playground center. My kids ask almost every day if they can go get babysat by the Y so they can play on the equipment. They they get a discounted price for any sports or lessons they sign up for during the year too. Members get first pick of class times and sign ups. Then the public get to sign up a week or two later.
I think a zoo would be awesome too. The one in OKC is pretty nice and I hear the ones in California are pretty cool too.
If there is a theme park nearby season passes would be wonderful for all of them. Of course that would probably be the most expensive. If I lived in southern California I would think it totally awesome to get season tickets to Disneyland... I will say though that if the parents are not on board or they have to buy their own tickets each time they may not want to go very much. My friends kids get season tickets to Silver Dollar City for Christmas from a neighbor but they didn't get to go to the actual park all summer because mom didn't feel well enough to take them plus she could not afford to pay the $45 day fee each time she took them. That's just a hardship on her. So make sure it is something the parents approve of if it's a commitment from them too.
What about a small vacation. My FIL gifted us a trip to Branson last month. All together is cost less than $1000. We got to do a lot more than the average vacationer because we went of the off season. It was still wonderful but totally cooler than hot hot summer, there were no lines at all at Silver Dollar City, the entertainment was perfect, and we had a wonderful time.
There are all kinds of ideas out there if you have the money. If you don't have much then I suggest you buy each child a gift card from somewhere fun like McDonalds or Walmart and let them go pick out something for themselves.