steel ones generally do weigh a bit more, just like bikes really
but considering you’ll be pulling 2 children inside them does it actually matter? No!
I think you will find the cheaper ones, say from Halfords, weigh a lot more than the more expensive ones.
Why the _____ would you want to pull more weight then necessary when you already have two kids to pull along?
Using your logic you might as well put them in the car and tow that along!