I took weekly piano lessons as a kid starting at age 9 until about 17. I didn't practice enough so I'm sure I didn't get as much out of it as I could have, but it was still a good experience and I'm happy that I learned to play, and I did go on to play clarinet in the high school jazz band, which I really enjoyed. I would say that at that age, if you don't care too much about how fast they progress, try the biweekly lessons and see how it goes. Have a set number of practices to complete between lessons, and see how they feel about it. My sister was forced into it, hated it, and dropped it early, but I tolerated it more and managed to keep it up and enjoy it. If the kids start to get better and start asking for more frequent lessons, go up to once a week but there's no harm in making sure they are actually committed to it before investing the extra time and money.