AndyP is right - the scaling is far from linear with weight. Anyway your reading has a fairly large margin for error.
But you can approximate your calorie burn by calculations (which is likely to be less inaccurate than a cyclocomputer if that doesn't take into account ascent, e.g. if solely wheel-driven). Even so the surface is important and is ignored by anything I can find. If you're on road or very smooth gravel, this will be good enough.
Easy option: upload to a fitness website
You could upload to route to Strava (free, but you have to register) for your weight, and then change the weight to get the values for other riders (you need to delete the ride, change the weight, and re-upload for it to update). I believe Komoot and RideWithGPS will also give you this information but I use those only for planning. This is what I would do, and the estimates are OK for casual riding, touring etc.
Tedious estimation option
You can estimate without Strava though. I'll assume a perfectly flat ride for now, then discuss how to account for a bit of climbing.
Using bikecalculator.com you can plug in your weight, riding position, tyres, etc. and the details of the ride. You'll get a number out for calories burn that will hopefully bear some resemblance to what your computer says. Then you can change the rider and bike weights to estimate for other people. If you choose, you can use these to scale your recorded figure, but I'd only do that if using a bike computer with altitude (ideally barometric; GPS altitude is poor in many conditions).
If the ride isn't completely flat, break it down into sections that are consistent within themselves:
- If you freewheeled down the hills, you can call that zero calories for the downhill stretches.
- Add up all the flat sections, and plug in your flat speed
- Add up the climbing. You may have to do this in multiple sections as not only do you want to get the right total, but also to use sections with a consistent speed. So perhaps a long draggy climb would be one section, and a couple of short steep bits another.
- Estimate separately, and add up.
Bikecalculator's metric trip calculator allows up to 6 segments with different speeds/gradients and the same rider/bike information. Note that it's not clear whether the calculator takes into account a difference in frontal area for smaller (lighter) riders; I would assume not.
Another easy option for the future
Use a mobile phone bike computer app for each rider. Plenty don't need a data connection and will run on old phones (e.g. relegated to kids' use). Giving the same details to an app and your computer will also give an idea of the spread in the estimates.
Buy a power meter for every bike, along with a suitable head unit (or phone app), again per bike. Stupidly expensive and unnecessary even for many of us who ride quite seriously (but don't race), let alone for a family ride.