Many people don't post their children's photos on social media at all; many people have tightly locked down social media accounts; many people post photos but nothing identifying and have anonymous accounts/tiny accounts.
Instamums have open, public accounts; they continuously post identifying photos of their children; plus they post identifying information about their homes, holidays, movements and children's nurseries/schools. (As well as potentially embarrassing content.)
It's child protection madness.
Apart from the safe guarding concerns, there are the potential issues of the children being teased or bullied over their parent's content; the possibility of mental health problems as the child ages and the attention disappears; and the plain old issue of the child being entitled to privacy, consent and ownership of their own digital footprint.
Why would any parent with their child's best interests at heart be willing to invite these potential risks into their family's life?
It boils down to child exploitation - the children are working to support their family.
The 'what about child modelling and acting' argument is a red herring, as the hours worked and income derived are tightly regulated. The private lives of child models and actors are usually kept private, because we all know how well it tends to work out for children growing up in the public eye.