That's my reading of Haranaharana Parva as well. I once discussed this at length in this thread we had in DBSK -
+* Pandava parivar *+The thought process went like this: when Arjun broke Draupadi-Yudisthir's privacy, the 2 may well have been in the process of procreating Prativindya. After that, it was unclear whether Arjun was exiled for 12 months or 12 years. I went on the assumption that he was exiled for at least 12 months, but had the option of staying away longer, depending on what he was doing. His travels to Naga lok & Manipura would have taken say a year or more, and then, after a one night stand w/ Uloopi, he'd have married Chitrangada and waited until Babruvahana was 1 year old b4 leaving. Again, it would have been a few months b4 he'd be in Dwarka.
Here, I tend to combine what's in Shrimad Bhagvatam and what's in Mahabharata. It's well known that Arjun introduced Krishna to his 4th wife Kalindi, and later helped him bring Satya home. Both Arjun & Bhima attended the swayamvara of Lakshmanaa, Krishna's 8th wife, and competed there. It's also known that Arjun taught advanced warfare to Pradhyumna & Satyaki, and maybe some of Krishna's other sons who were old enough at the time - maybe the sons of Rukmini, Jambavati & Satyabhama.
Following all this, Arjun would have had his tryst w/ Subhadra, which would have ended w/ him & Subhadra entering Indraprastha accompanied by all the Yadavas - Krishna, Balarama, Pradhyumna, Satyaki, et al.
Now, in this time frame, it's reasonable to assume that since both Y & B had priorities ahead of A as far as Draupadi went, it's more likely than not that Draupadi would not only have given birth to Pradhyumna, but would also have at least been pregnant w/, if not given birth to Satsoma. I think it unlikely that she'd have moved on to Nakul or Sahadev b4 she served Arjun. Which is why, I think she alternated b/w Yudisthir & Bhima until Arjun's return. Chances also are that while she was w/ Bhima his first year, Yudisthir married Devika, and while she was w/ Yudisthir his second year, Bhima married Valandhara. None of the 'not bringing their wives to Indraprastha' business.
Now, as far as Abhimanyu went, he was just 3 years old when the Pandavas left into exile, so my theory is that once Arjun returned to Indraprastha, Draupadi immediately got him, and Subhadra didn't have Abhimanyu until near the end. Draupadi would have then become pregnant w/ Shrutakarman, and later moved on to Nakul & Sahadev. Later, Nakul & Sahadev would have married the daughters of Jarasandha & Sishupala. I'd imagine that a few years would pass b/w Rajasuya yagna and the game of dice. Towards the end of that period, Abhimanyu, Nirmitra and Sahadev's son from Jarasandha's wife would have been born.
So the upaPandavas would have been around and young, but not as young as Abhimanyu.
Exit question: why is the term 'Upapandavas' reserved for Draupadi's sons alone, and not cover Yaudeha, Sarvaga, Abhimanyu, Nirmitra, Suhotra at least? Even Iravana & Ghatotkacha ought to be included - their being out of Indraprastha is one thing, but that doesn't make them non-Pandavas.