Each Forest and Saproling will have the card types creature and land, and the subtypes Forest and Saproling. Each subtype maintains its correlation to the proper type: Forest isn't a creature type and Saproling isn't a land type.

Each Forest has the ability "{T}: Add {G}."

Saprolings enter the battlefield as land creatures, and abilities that trigger whenever a land enters the battlefield will trigger.

Forests enter the battlefield as land creatures, and abilities that trigger whenever a creature enters the battlefield will trigger. Because you haven't controlled these creatures since your turn began, you can't pay the {T} cost of their mana ability.