Is seniors or majors higher in the little league?

Seniors is for older kids. The official levels are t-ball, minor, major, junior, senior, big. Levels are age-based, with some overlap except between major and junior. Minor league is also typically divided into roughly age-based sublevels.

That being said, what you're looking for is the bat regulations. How about a quote from the LL rules?

"1.10 – The bat must be a baseball bat which meets Little League specifications and standards as noted in this rule. Senior/Big League it shall not be more than 36 inches in length, nor more than 2 5/8 inches in diameter, and if wood, not less than fifteen-sixteenths (15/16) inches in diameter (7/8 inch for bats less than 30”) at its smallest part. The bat shall not weigh, numerically, more than three ounces less than the length of the bat (e.g., a 33-inch-long bat cannot weigh less than 30 ounces). All bats not made of a single piece of wood shall meet the Batted Ball Coefficient of Restitution (BBCOR) performance standard, and such bats shall be so labeled with a silkscreen or other permanent certification mark."

To sum this up, you need a -3 BBCOR bat to play LL Seniors.

Seniors are higher. It goes

Tee ball

Instructional

Minors

Major

Juniors

Seniors (juniors in my league is just the younger ofthe seniors so 14/15)