Hello, I'm a uni lecturer and I have some responsibility for the workload of the staff in my subject area. I also have 2 DC, one primary school-age and one pre-school.
Workload models will vary from institution to institution and probably from department to department within a given institution. The UCU also isn't very in favour of workload models so it may be a contested issue in your uni. But ask to see whatever passes for a workload model or guidelines, to see if you are being hard done by. it does no harm to make sure that you are not being put upon by some lordly professor who has creamed off a small amount of the best teaching for himself (it can happen!). But don't forget the workload will also take into account admin roles, so if your admin roles are quite junior, your teaching load may go up to compensate.
Tbh the job is very hard on new academics, as you are having to learn the ropes of teaching as well as trying to maintain a research profile, and quite possibly you might be asked to fit in a staff development course for new lecturers too! But it is not at all uncommon to be asked to teach something within your subject area but outside of your specialism - what level of study is it? If it's only first year level, then it could be a survey course with only foundational knowledge. It's OK to go on the learning journey with your students together and be a facilitator, teaching them about the context of your subject, helping them with study techniques and academic skills, and quite OK to admit to a class you don't know something. Also, one of the best ways to learn about something is to teach it, doing some reading for the course might open up interesting new avenues for your own research.
Module design is also an excellent skill to have, and your uni may have staff development workshops on this - see it as your chance to transmit your enthusiasm for your subject to your students.
As for the research balance and work-life balance, I hear you, sister! Do you get a research day? If so, be absolutely ruthless that you use it for your research and be shameless in telling colleagues that other stuff will just have to wait . Likewise, be shameless in maintaining your time with your family. I refuse to work weekends apart from the occasional conference, and in the evenings I don't do anything apart from simple marking or a little bit of reading for the next day. It does mean I may not be on a fast track to professorship, but I have to be more circumspect about my career and think long-term rather than short-term.
Hope this helps a bit!