Hi DelGirl
I can't help with suggesting accommodation as I'm up north, but I can help a bit with the rest. Firstly, when were you thinking of coming? I ask because if you're looking for language tuition and nursery or even just basic childcare you'd do best to avoid this period as everything shuts down for August.
Also, how old is your DD? If she's 3 or over she'd have to go to a scuola materna as opposed to nursery (nursery, or nido, is for under 3s only) and I'm not sure even a private one would accept a pupil for only 3 months, plus please bear in mind that in the smaller towns at least most private kindergartens are Catholic and quite nauseatingly religious (don't know if that would be a problem for you). You'd be better off getting a babysitter, perhaps a student who can look after her in the afternoons or something.
Re language tuition for you, most local councils run courses for foreigners, but they tend to start from September and last through to May/June. Otherwise, there is always private tuition but it's expensive. In most major cities and some not so major ones there are loads of language schools, so you should be able to take your pick.
Other than that I don't think you'll need any particular documents if you're only staying 3 months, but I may be wrong, the bureaucracy in this country is staggering. If you do decide to stay prepare yourself for the thousands of pieces of paper you're going to need just to have access to basic services (eg. residency, permesso di soggiorno, codice fiscale etc.). Are you thinking of working here should you decide to stay long term? If so, and even more so if you're thinking of starting a business locally, beware of all the taxes. If you think the UK has high taxes you ain't seen nothing yet.