Alright this is turning out to be longer than expected I'll have a TL;DR at the bottom but I'd recommend reading everything if you want the best picture of what's going on or want to tweak things yourself.
So, flow rate is area of the pipe or channel times the velocity of the liquid. Velocity of your liquid is going to depend on the angle you place the bucket (I think), but let's say it's flowing at roughly the speed of a river. That goes from around 0 m/s to 3.1 m/s according to google, so that's what we've got to work with. We can always bring it higher by saying the bucket has a sort of suction function, so if it turns out that we can go higher we can always do that. Or just make the thing bigger like you mentioned.
So, we need to get the amount of force a 15ish year old boy could handle. If we listen to an answer from stackoverflow, the force of moving water on a wall would come out to F = pAv^2, where p is density (for simplicity's sake we'll assume 1. It's a swamp so it's likely higher than 1 but I'm not finding any average densities of swamp water soooo), A is area, and v is velocity. And, since we control area and velocity we can make our force basically whatever we want.
Unfortunately, there isn't much information on how much a child can lift and hold, and I'm not exactly sure how to calculate that. So, I'm going to be doing some synthesis and guesswork and I can't guarantee accuracy of this next bit. I'm going to be going off deadlift standards and then, since we're assuming holding it in place for a while, I'll be subtracting a decent amount off of that to assume the amount that kid can keep in place.
Deadlift standards are, apparently, based off of body weight. I'm going to be doing all my calculations in metric units because it just makes the math a lot simpler not having to convert back and forth, so keep that in mind. The average weight of a 15 year old boy is 56 kg, but since we're assuming he's towards the scrawnier side, I'll just call it an even 50, which is roughly the average for a 14 year old. A man weighing 50 kg has a standard deadlift of 44 kg if they're an absolute beginner (which I'm going to assume since he was an office worker).
Let's bring that down to 35 kg to assume holding in place. That's 343.35 Newtons. Let's assume velocity is at that maximum of 3.1 m/s we saw earlier, so if we divide our force by that our target area would be 35.72 m^2, or a circular object with a radius of 3.3 meters, and that's kind of a yikes if we're looking at a bucket or something. So, we'll go ahead and bump up that 3.1 m/s, say double it. This brings our target area all the way down to 8.9 m^2, or a circular object with a radius of 1.68 m. That's still huge for a bucket, but if we assume some sort of large box or something we could do a square container with a side length of 2.9 m.
Of course, I think it's easier to increase flow rate and assume magic shenanigans, so let's just make it a solid 10 m/s. That's super fast, and brings our theoretical bucket radius down to 1.09 m. If you want a bucket with a more reasonable (but still pretty large imo) radius that's a flow rate of 20 m/s and a radius of .52 m.
That gives us a flow rate of 16.989 L/s. Let's assume a small key area of 100m x 100m with an average height of ~85 cm (half of the height of an average 15 year old boy). Assuming the swamp is a perfect cube (which it's not), which is an area of 8,500 L. That'll take almost 500 seconds even to drain, so honestly not that bad.
So, TL;DR:
Considering average weights, a teen boy could maybe hold a force of 343.35 Newtons for a decent while and if you want to utilize all of that, you either need a big container or some sort of suction power on it. Considering a suction making the speed 20 m/s that's a bucket with radius .52 m (which is still decently big imo. Roughly a foot and a half). It'd take just under over 8 minutes to drain a fairly small (100mx100m) chest-high swamp.
If you have any questions, please ask, I can explain more or do some more tinkering for you!