it's entirely up to you (of course) - i worked as an ambassador at a UCL postgrad open day recently, and i'd say around 40% of students had brought a parent along. i think it is entirely dependent on your reasons for it though. if i took my mum on open days (which i'm planning to!), it would be because i wanted her to see the places i could be staying in next year, and share this new exciting part of my life with her. if you think you might end up relying on her too much to ask questions etc, or if you think you won't talk to other applicants/tutors at the open day because she is there, i'd take the opportunity to go alone.
it's also entirely dependent on the timetabling of the open day as well, i think - if there are taster sessions etc going on, it probably would be a hassle to bring your mum, and probably quite boring for her, whereas if the day will be spent in talks and tours around the campus that would be a bit more accessible.