My parish has Bible Study every two weeks on Sunday nights. We have a pretty large turnout for a Bible Study.
However, in my experience, the issue is that most (not all) Parishes tend to scheduled events that work well for the people who typically come… the older parishioners.
Older parishioners typically do not have young children at home, etc.
I personally believe that we Catholics should take a page from many Protestant communities by offering free babysitting during adult faith formation programs and devotions.
Babysitting could either be a paid job or even better a ministry. Could even include some free & fun Catholic activities during the babysitting.
When there is truly a family event at my parish, a large number attend. But those events are usually just fellowship, but a younger family can bring the little ones.
I truly believe that we offer free babysitting at parish events, more younger families will attend.
ALSO - this is also why formed.org is such an outstanding resource for parishes. With FORMED, parishioners can take part in studies at home, watch Catholic movies, etc. We find that the more a parishioner uses FORMED, the more involved they eventually become.
Finally, I would like to remind people that unlike our Protestant brothers & sisters, the Catholic Church is larger than just our Parish. Our Diocese is technically our local Church. I know many Catholics who are not really involved with their parish, but are involved with things across the diocese. They are members of Opus Dei, third orders, and/or diocesan or regional Catholic groups focused on spirituality, prayer, chastity, God in the workplace, etc. So many Catholics receive Catholic fellowship, etc outside their parish too.