The weigh attach to the balloon is too light therefore, it raised/flew up with the ba

[scott833]

1) The weigh attach to the balloon is too light therefore, it raised/flew up with the balloon.
2)The little boy is not strong enough to hold down the big kite, as a result he flew up/raised up with the kite.
3) The little boy is not strong enough to hold down the big kite, therefore when the big kit flew up, it took him with it.

[Barb_D]

1) The weigh attach to the balloon is too light therefore, it raised/flew up lifted up with the balloon.
2)The little boy is not strong enough to hold down the big kite. As a result, he flew up/raised up with the kite. -- I'm feeling iffy about this one.
3) The little boy is not strong enough to hold down the big kite. Therefore, when the big kit flew up, it took him with it.

1 is how it sounds more natural to me. Others might find your originals okay.

I'm not sure how I feel aout #2. At first I was doing to say that none of them worked. Maybe it's okay.

#3 is your best one, grammatically, though I am doubtful about it factually.

[Raymott]

1) The weigh attached to the balloon was too light. Therefore, it rose up with the balloon. (You might mean "was too light")
3) Also, perhaps, "The little boy was not strong enough ... "

In both cases, it's irrelevant that the weight hasn't changed, or the boy's strength hasn't changed. We'd usually still use the past tense.

[Barb_D]

I didn't even notice the attached, and is/was. Thanks for not letting those slip through the cracks.