If the Joint development agreement between land owners and developer is duly registered then the GPA executed by the land owner in favour of the developer must also be duly registered. Apart from the above, after obtaining the sanctioned building plan, the land owner and developer would have executed amongst them a notarized Sharing or supplementary agreement, evidencing the division of flats amongst the land owners and developer in their sharing ratio mutually agreed upon in the JDA. This is the standard procedure followed in Bangalore.
Now coming to your question, father has gotten the GPA from his two children as the guardian, but he and his wife have executed the GPA in favour of the developer to sell the developers constructed area in favour of intending purchaser/s without their direct involvement, which is completely legal and can be accepted. Assuming that the children are minors, he being their natural guardian can take a GPA from them and represent them along with his wife.