Good for you OP :) I work at a university with plenty of mature students and would echo the advice of many others about carving out specific study time for yourself.
What you should check out on the open day -
-What kind of specific support do they have for mature students - i.e., do they have a dedicated mature student rep? Can you use the campus nursery for the 3yo? My uni would let you do both those things, but not all do.
-How far in advance do they tell you about class schedules? Technically f/t students should be available Mon-Fri, 9-5, but real life might not work like that for you. Plus schedules change semesterly, so whereas you might have a 9 am session every Tuesday your first term, next term it might be completely different. How will this work with your childcare? Will they be flexible about putting you in a different group if one won't work out for you?
-What kind of 'flexible learning' facilities and/or online resources do they have, as these are often a godsend for students with children? I.e. our campus library is open 24/7 now so a LP has much more flexibility about studying in the evenings, say. We are just starting up with electronic submission of essays so you don't have to plan a special trip in - this has been a problem in the past for my mature studs, who have to get a neighbour to watch the kids or whatever just so they can hand coursework in. We also have many books online and other library facilities so you can more easily study from home.
-How many hours per week do they expect you to study per module? I.e., our students only have c. 9 hours per week contact time, which sounds a doddle - till you realise the amount of reading and written work they have to do equates to approx 45 hours per week!! So check that out, otherwise you will be megastressed. Make sure you actually have that amount of time available in your week.
-Also, will you have a personal or pastoral tutor to talk to if you have problems? Some universities are more 'with it' on this score than others. Personally I have a couple of mature studs who come to me regularly to get support as they feel a little more unmoored having come back to education. This can be a lifeline.
- Finally, will those around you support you? One of the biggest gripes of my women mat studs with DC is that neighbours/ friends/ family etc don't really 'get' the amount of effort a degree takes and will take the mickey asking them to 'just come out tonight' or expecting them to babysit at short notice if they are home studying, etc. Be clear that work time is work time and you can't combine it with tidying the house, minding the NDN's 6mo, etc. Make sure others around you don't regard it as a 'free' time.
hth - good luck and do pm me if you have any questions :) hope you go for it; mature students are a godsend in general as they work so hard and really are dedicated. Your future tutors will be lucky to have you, I'm sure.