1. Prathivindhya, 2. Shruthasoma, 3. Shruthakeerthi, 4. Shathaaneeka and 5. shruthasena were referred to a Upapaandavas as they were the five sons of the five Paandavas born to them from their common queen Droupadi.
[ They were often referred to as Droupadeyas(=Droupadi's sons) in the epic. We use the word Junior Paandavas in this document.]
( Another king mentioned by Drupada- in Udyogaparva- was also named Prathivindhya. One of Viraata's sons and one of Viraata's brothers too were named Shathaaneeka. Be careful to distinguish among the people with same name.)
Paandu's five sons Yudhishthira,Bheema,Arjuna,Nakula and Sahadeva were collectively called Paandavas (Paandu's sons) or Paarthas( Sons of Pritha i.e. Kunthi). Also,the last two, thins they were, were called Maadreya (Maadri's sons) as their mother's name was Maadri.
The five paandavas had a common queen Droupadi from whom the five had one son each, namely, Prathivindhya,Shruthasoma, Shruthakeerthi,Shathaaneeka and Shruthasena respectively. These were referred to as Upa-paandavas ( meaning, junior Paandavas). They were younger to their celebrated cousins Ghatothkacha and Abhimanyu. During their fathers' exile, these five spent that period at Dvaaraka alongwith Abhimanyu. They learnt elements of archery from their father but most of their practice was obtained from Pradyumna ( Krishna's son) and Saathyaki ( Krishna's cousin).
They fought in the war for their fathers' cause and proved their worth in the war as fitting sons to their fathers.They were attacked while asleep,during the night of eighteenth day by Asvaththaama and were eventually killed.
In Maarkandeya puraanam, there was an explanation for their untimely death as unmarried. They were actually the gods caled vishvas who took birth as mortals due to a curse from the irate sage Vishvaamithra . As a redressal, they were allowed to stay in the mortal world for a short period only and they wouldn't have marriages or offspring.