The OP appears to be US based. If so, the benefits of US pre-clearance will be largely negligible.

Unless the flight schedules / layovers on KLM do not make sense, I would encourage you to go KLM assuming the fares are broadly similar to EI. I think most would agree that an A330 + ERJ combo is preferable to an A321neo + ATR, especially in Y. KLM is a solid airline with a good onboard product in Y and will look after you in the event of disruption. EI has a patchy record on customer service during disruption. Given there are connections involved and you are a relatively new traveler, you need to be confident that you and your family will be supported if anything goes wrong.

In terms of cabin baggage on EI, you should have no issue on the A321neo, but anything larger than a medium-sized backpack will not fit in the overhead bins, or under your seat, on the ATR. These bags will need to be checked-in at the gate in DUB pre-departure. The same is likely on KLM with the ERJ.