Generally speaking boarding starts about an hour beforehand and queuing time should not be much more than 40 minutes in the port. You would need to keep in mind that the route to Holyhead can be a bit busy during the daytime. This is a bit generalised, it is very much dependent on how busy it is on the day, especially during the holidays.
I would advise you to contact the ferry operator
directly as well, as they may be able to facilitate quick boarding with Sam and help make your trip go as smooth as possible. Keep in mind that if you do make use of the ferry company's assistance options they may want you to arrive earlier, which might make the wait time a bit longer. It's very much worth your while to give them a call to see what they can offer!
There is indeed wifi on board of most ferries nowadays. Stena Line has onboard wifi for free
, and Irish Ferries
does too. You'll have to keep in mind this free wifi is only in specified locations onboard. If you want to be sure you have certain games, series or films with you to keep the children occupied I would advise to download them beforehand if you can, as "maritime wifi" can be a bit spotty at times.
What time of the day are you looking at travelling?