The man's dress style is an 1870's fashion but did carry over well into the 1880's.
The woman is a different matter,she is wearing an 1880's style,if this was 1870's
I would expect to see a more elaborate dress/hair style.Her "V" or waistcoat shaped
bodice is an early - mid 1880's creation and her plain neckline mid - late 1880's.
The infant may not be a boy,usually holding flowers indicated a girl,also his/her
strappy shoes didn't appear untill well into the 1880's I believe.It would also
be rare to see an outdoor family photo from the 1870's.
It would be of some help if we could see all of the card plus reverse if you have the original.
Until then I would suggest a date of late 1880's possibly into the 1890's when these
outdoor shots became more common.
jim