[broken link removed] (ignore the 2006 year on this - you can just annotate it if filing for 2007) would be the form but you don't necessarily have to file a full return to claim the credits/allowances to which you are entitled. Many of them can be claimed online either [broken link removed] or by registering for their [broken link removed]. This thread might also be of interest:
There are no credits for having kids but if one spouse stays at home to mind them then you may be able to claim the Home Carer's Tax Credit. Obviously you can also claim Child Benefit and Early Childcare Supplement (for under 6s) payments from SW.
Why would you want this at this stage? The 2007 tax year ends on 31/12/07. The information you need to enter on the form (P60 details etc) will not be available until that date, or possibly a little later.