。
The baby was splashing around in the bathtub.
Literal
Baby [subject-が] bathtub [inside-の中で] splash-splash was-doing-[continuous-ていた].
ぼちゃぼちゃ is an 擬音語 for the sound of water being splashed or slapped around. The verb やる here stands in for a more specific activity verb — a very common pattern where やる (or する) pairs with a mimetic to mean 'to make the X sound/motion.' ~ていた gives the past continuous.