If you read the guidance or the information on the UKBA website *carefully*, you'll know for sure on you own!!! See
How you can switch in to the dependant route. Note that you can apply for family members to switch to Tier 1 dependent status only if they are in UK as your dependents.
So (as far as I understand), if you wish to apply for your child as a Tier 1 dependent alongwith your own application, then he must have a current leave as a dependent. To get a WP dependent leave for your child, you'll have to shell out 395 quids, so there is no net savings (contrary to what is suggested above) even if you apply for your child's dependent status along with your own Tier 1 application.
On th other hand, if your wife is here as a WP dependent, then you can include her application with yours and save the 395 quids for (her) dependent application.
Call up UKBA customer care. Number available on UKBA website.
regards