Hello We call it rampant caries they usually spread very rapidly from tooth to another. It will better you get the tooth treated

Cavited tooth not cleaned by brushing.you should visit dental clinic especially pedodontist and restore the tooth with restorative material. if there is prolonged bottle feeding during baby is sleeping should be avoided. Dont give snacks between meal times( better to give snacks immediate after food taken). After taken food ,child should do mouth rinse with water to prevent accumulation of food in teeth. In dental clinic after restore the tooth.ask dentist to apply pit and fissure sealant or flouride application to control caries in remaining tooth