Hi Louise and welcome aboard! Lisa and Beth have already provided some great advice and now it is my two cents worth
It is a shame that you have such a short period of time to stay there, but one day is better than no days at all
As the others have mentioned, there are two theme parks in the Disneyland Resort, there's the original Disneyland (still the best theme park in the world, even though it is 55 years old!); and immediately next door is Disney California Adventure (which is on the site of the old Disneyland car park). Even with two days, I'd probably be tempted just to spend both days at Disneyland, mainly because of the construction that is happening in DCA these days (they are expanding big time!) - although DCA does have the nightly "World of Color" show which is spectacular.
You will be visiting Disneyland during the Halloween Time festivities, but don't let that concern you - Disney do halloween with a sense of fun and lightheartedness, scares are few and far between.
Definitely head up to Fantasyland when you are there - and don't miss the Princess Fantasy Faire.
As Beth mentioned, you can use the child swap facility if you and Mr Louise both want to go on rides that the little one can't manage (Indiana Jones, Space Mountain etc.). Don't miss the nighttime spectacular "Fantasmic" on the Rivers of America each evening as well!
Wow - there is so much there it is difficult to put it all in words
As for accommodation, we always stay at one of the Harbor Blvd hotels, immediately across the road from the park entrance. The prices are much cheaper than the Disney hotels. The Harbor Blvd hotels are just like an Australian motel that you'd find in a seaside town here - not flash, but adequate.
You might find you can get a bargain rate at the normally expensive Disneyland Hotel at the moment as they are renovating - however note that the renovations are MAJOR and the entire outdoor pool and garden areas etc are closed. The other two Disney owned hotels are the Grand Californian (which is brilliant, has a great location, but very expensive) and the Paradise Pier Hotel which is cheaper but a long way from the Disneyland entrance.
My recommendation is to go to one of Harbor Blvd Hotels near the park entrance: the Tropicana; the Carousel; Best Western Park Place Inn and Mini Suites; Park Vue Inn; and the Desert Inn. They are all in a row. I think my favourite has been The Tropicana, but my partner says she preferred the Carousel - so each to their own I guess!
Keep asking questions - we've got endless advice around here and are not afraid to share it
We'll have you fully prepared by October! When do you leave Australia?