This can quite easily be performed using PowerShell. It is, however, an absolute requirement for this solution that the 50 subsites are site collections (as Darko suggested, too):
1. Create a content database
New-SPContentDatabase -Name <db_name> –WebApplication <app>
2. Determine disk space used by a specific site collection
$used = (Get-SPSiteAdministration -Identity <sitename>).DiskUsed
3. Move site collection to another database
Move-SPSite -Identity <siteName> -DestinationDatabase <db_name>
Pre-Reqs: Db must be attached to the same web app and physically on the same server
Post actions: iisreset required.
Let me know whether this did the trick.