Good Folk song in there somewhere:-

For he is only 13 yrs,
And I am just 15.
Oh the bonny boy is young,
But he's growing.

I guess like every other situation that crops up in life that causes surprise and doubt etc.... we will just have to wait and see... and I reckon the authorities will be monitoring them to some degree (if they have the resources).
I cared for teanage girls and their babies for over ten years of my career... we had them from as young as eleven (made pregnant at ten by her own father) to eighteen. Most of them arrived pregnant and had their babies at the local maternity hospital....some were brilliant mums... some were good mums... some weren't capable of caring for themselves, never mind children... but often it was the ones we thought wouldn't cope that coped quite adequately with being a mum. It was hard for us to assess who would keep their babies and who would give them up.... so how can anyone judge whether this child will be loved and cared for by its parents.. whatever age?. It is not impossible for a thirteen year old boy to develop a sense of responsibility beyond his years and how often has it been tested.
I guess the parents will have responsibility for all three of them for the time being... I hope they are well enough equipped to 'help' in the bondings of all three.
Once again, let's just wait and see.. because if it doesn't work out, we will hear more of it.. if it does... great..... and I wish them all the joys in the world.