I have found following the different characters in this case confusing, since there are so many. So here is what I now understand, based on all of the updates from Clarin (news) from their recent daily updates.
Desaparición de Loan Danilo Peña en Corrientes, EN VIVO: la búsqueda del nene de 5 años y las últimas noticias del caso este 26 de junio
At the lunch:
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Catalina Pena. 87 years old. Loan's grandmother. Location of lunch near the small town of Nueve de Julio.
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Laudelina (Catalina's daughter; Loan's aunt) & husband,
Antonio Benitez. Antonio has been detained. LE has requested a 'genital sample' (not sure if this means semen or pubic hair).
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Monica Millapi (detained) & husband
Daniel "Fierrito" Ramirez (detained). Monica was invited by Laudelina.
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Maria Victoria Caillava (detained; former municipal employee of 9 de Julio) & husband
Carlos Perez (62; detained; former ship captain; alleged mastermind of disappearance). Catalina, the grandmother, said in an interview that she invited Maria although Loan's father said they weren't invited. Maria and Carlos drove to Chaca in a car that contained traces of Loan.
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Loan Pena and his father
Jose Pena. (not clear who invited them or if Jose just randomly decided to go, as stories conflict.)
ETA: (adding this section since I missed mentioning another person detained.)The other person detained, but who was not at the lunch, was
Walter Maciel, a commissioner (LE) for 9 de Julio. He is charged with interference in the investigation. It's not clear in what way he interfered, however Loan's father said that Maciel told him that Loan was seen the day after he went missing.
Everyone is pointing fingers at the others. Multiple people (including Loan's mother) are saying that
everyone from the lunch should be investigated, including those not detained. Note that the only people at the lunch that weren't detained were Loan's 87-year-old grandmother, his aunt and his father. None of the detained who have spoken through their lawyers appear to still be saying that Loan just wandered off.
I found it odd that Loan, at 5 years old, had never been to his grandmother's house before.
I also found it odd that such a large group of non-family members would end up at the grandmother's house for lunch considering she is 87.