When Pandavas 13 year period of exile started, some of their children were already born whereas some were born in the woods (Upapandavas). The Upapandavas were born in the woods and were raised in Panchal. What about the other's like Abhimanyu and other children of Pandavas from their other wives?
Abhimanyu and his mother Subhadra were in Dwarka. And as per Mahabharata: Vana Parva: Markandeya-Samasya Parva, sons of Draupadi also came from Panchala and joined Abhimanyu in Dwarka where they were trained by Pradyumna (son of Krishna and Rukmini).
And Krishna also said, 'O Krishna, O daughter of Yajnasena, those sons of yours, are devoted to the study of the science of arms, are well-behaved and conduct themselves on the pattern, O Krishna, of their righteous friends. Your father and your uterine brothers proffer them a kingdom and territories; but the boys find no joy in the house of Drupada, or in that of their maternal uncles. Safely proceeding to the land of the Anartas, they take the greatest delight in the study of the science of arms. Your sons enter the town of the Vrishnis and take an immediate liking to the people there. And as you would direct them to conduct themselves, or as the respected Kunti would do, so does Subhadra direct them in a watchful way. Perhaps, she is still more careful of them. And, O Krishna, as Rukmini's son is the preceptor of Aniruddha, of Abhimanyu, of Sunitha, and of Bhanu; so he is the preceptor and the refuge of your sons also! And a good preceptor, would unceasingly give them lessons in the wielding of maces and swords and bucklers, in missiles and in the arts of driving cars and of riding horses, being valiant. And he, the son of Rukmini, having bestowed a very good training upon them, and having taught them the art of using various weapons in a proper way, takes satisfaction at the valorous deeds of your sons, and of Abhimanyu. O daughter of Drupada! And when your son goes out, in pursuit of (out-door) sports, each one of them is followed thither by cars and horses and vehicles and elephants.'
Other sons of Pandavas didn't join them even when they were ruling in Indraprastha:
- Ghatotkacha and his mother didn't join Pandavas and promised to come when called:
"Then Hidimva, knowing that the period of her stay (with her husband) had come to an end, saluted the Pandavas and making a new appointment with them went away whithersoever she liked. And Ghatotkacha also--that foremost of Rakshasas--promising unto his father that he would come when wanted on business, saluted them and went away northward.
- Ulupi also stayed in Nagalaoka and didn't go with Arjuna. ~ Source: Babruvahana in Mahabarata
- Chitrangada was also putrika and her marriage with Arjuna happened on the condition that their son will be ruling Manipura as Chitrangada's father had no son. ~ Source: Babruvahana in Mahabarata