The landlord has to make 'reasonable' alternations or adjustments to the property to allow for your disabled status. It also has a higher duty of care towards your family because of the age of your daughter - if she is in physical danger from traffic when playing in your garden then that would certainly be an issue they should take up.
So you may have a case under the Disability Discrimnation Act- but you'd need detailed legal advice on this, so I would consult your local Housing Aid Centre or Law Centre.
But I would also suggest talking to your local Home Improvement Agency (sometimes called 'Care & Repair' or 'Staying Put' services). It may be - I'm no expert on this question - that they might be able to advise you on the availability of a Disabled Facilities Grant or some other scheme which would allow either you or the HA to pay for the installation of a fence at little cost to yourself.