In that case the answer is yes she will need her own application.
It's somewhat unusual route to take for a fiancee when a child is involved.
However, in principle there is provision in the UK immigrtaion rules for children of fiancee's
Here's what UKBA state:-
In assessing whether there are serious and compelling considerations it would normally be undesirable to exclude a child who had largely been cared for by the 'fiancé(e) / proposed civil partner parent'.Entry clearance with a view to settlement may be granted to a minor dependent child of a parent who has been admitted, or is seeking admission, as a
fiancé(e) under paragraphs 290 to 295 of the Rules provided the following additional requirements are met:
the child is under the age of 18, is unmarried and is not leading an independent life nor formed an independent family unit;
the child can and will be maintained and accommodated adequately without recourse to public funds with the parent admitted or being admitted as a fiancé(e)
or proposed civil partner;
there are serious and compelling family or other considerations which make the child's exclusion undesirable;
that suitable arrangements for the child's care have been made;
there is no other person outside the UK who could be reasonably expected to care for the child;
the terms of any custody order relating to the child do not prevent the child being taken to another country with a view to settlement there.
Concerning application forms, yes, you'll need to submit two forms VAF4A, one for your fiancee visa and the other for the visa application of your child as a minor dependent of YOU. This also means payment of 2 visa application fees.
You will need to declare your child on your application and also submit one for the child.
If granted then both visa's will be issued for 6 months duration only and during this time you will be expected to get married and submit an application of FLR for yourself and your child.
As I said, it's an unusual route.......is there any special reason to follow this?
Finally, please be informed that your fiance/sponsor will need to meet the UKBA Financial Requirement at a higher income threshold of £22,400 for 1 child in addition to the partner