Just an FYI, but you should have a look at Totcycle and then if your really bored my blog. Totcycle have done a great review of front mounted seats.
But realistically you need to transform your bikes or buy new ones. Consider 2nd hand for hybrids etc.
Help needed for child's bike seat please
Re: Help needed for child's bike seat please
I hate snow.
Re: Help needed for child's bike seat please
I still think purpose made bikes are going to work better and be safer. Straps, bolts etc can snap and shear off. We're talking about kids here and you can't get them back if they have a fatal accident.
Something like this is ideal. Bakfiets long cargo bike.

You can even book a test ride at Practical Cycles (I have no connection with them btw).
Something like this is ideal. Bakfiets long cargo bike.

You can even book a test ride at Practical Cycles (I have no connection with them btw).