Network gurus, a little help please...

gagaga

Just to be clear about the talk of bonding.  'True' bonding - using more than one connection to provide a single, faster one - needs the ISP to set this up as ultimately the traffic needs to end up at the same router on their network.  So you can't mix (eg) BT VDSL and a 4G line.  What you can do with Draytek/Asus (iirc) and others is have two separate connections.
I ran 2 VDSL lines for a while - got a deal that meant running a Sky and a TalkTalk line was cheaper than the 1 BT one I had. Actually stuck a 4G stick in the router for a while too, just because I could..
You've got two choices - as people have said you can spend time saying traffic X goes to router A etc, but unless you do this stuff for fun, you'll probably just leave it in auto.  What it'll do when new traffic arrives is stickily direct all that type of traffic (eg Netflix) from that device to one of the connections - so if you start watching Netflix on an iPad , it'll keep sending traffic from that watching session to the same place.  It won't use both connections at the same time for the same service.
So..  you won't get a total bandwidth of A B - you'll get the maximum speed of whatever connection is chosen for that connection at the start.  However for your scenario of a family, two balanced connections would work well as it would naturally stop one user hogging all the bandwidth.
Unless you'd got huge problems, I'd get a balancing router for the VDSL 4G and sit tight for a year or two - lots of changes in broadband happening (lots of grants now/coming) and chances are in the next year or two you'll be able to get true fibre fitted for sensible money, or 5G (which will be 100s of Mb) and that's even before one of the several satellite constellations planned come on stream (lookup the bandwidth on those, at launch they will be able to provide 2-300Mb for every person on earth).