This story shows your caring nature and your will to look after children you see as being in need. Yes I am sure that witnessing this regularly must be very unnerving and I too would want to do something. I think I would be cringeing all the way home!
I worked in children's homes with suffers of abuse; so I hope I can provide some insight. The difficulty with neglect/emotional abuse cases is that they are notoriously subtle - so getting real 'evidence' is very difficult - even when you are professionally involved with the families. Some of what you see seems consistent with neglect - children acting up to get attention for example. However; you are not privvy to full information of family circumstances outside of the bus journey. Cases often move up the priority scale when behaviours are seen for a number of months - by which point people are asking what new strategies have been introduced to discontinue any decline. Just explaining the legal climate. Not one 'abuse' case is the same as the next.
Sometimes is not about trying to work out the reasons that children are growing up like this (poverty, lack of parenting skills, lack of washing machine, non-bonding, parents own history of abuse) - families operate to all kinds of different standards. Its just about saying - this does not feel/look right and it is generally unacceptable to have children live like this. I think you have something when you say that the kids have stopped listening to their parents' shouts - I call it 'glazed' expression.
However; at the moment you do not have enough factual info. to even report what you see - bus and first names won't help any agency identify the children properly. You are also not in the position to ask the family questions (I can imagine how that would go down on a bus!). Any agency that might pick this up would be asking how you came to your impressions of the family. They will need to separate the physical evidence - threadbare, filthy clothing, children ignoring parents and your impression of what is happening (which they would call circumstantial evidence).
If you continue to be worried; I would suggest a call to Childline/NSPCC or other such aids to children. It will be a chance to unload; and also discuss their procedures for reporting and what evidence is needed. The internet also has basic child protection procedure info.
The children are all under school age and I imagine not being seen by a health visitor/G.P. But once they get to school/other childcare service; they are likely to be picked up on.
In the meantime; if there is any escalation/physicality - you can ask the bus driver to stop, make out he/she has had a call on his/her phone (or other cover story) and ask him to call the police (first call for Emergency Protection Procedures).
Because cases like these are hugely emotionally demanding and legally complex - I would always suggest that professionals in that area of work deal with it - should reporting become possible.
It must be very frustrating to sit there and watch- but its hard facts that often get the process of Protection started.
Every empathy for the children right now though. If they are being subject to what you suspect -truth always outs!