I know that because of kibud Av, it is proper that a child pay the debts of their parent who was niftar. Does the same apply to verbal commitments to purchase something that the parent died before they completed the transaction? Since this would be a case of mechusar amana and the niftar was an oines perhaps there it is not the same as an actual debt that they had?
As you note, for buying an agreement to buy an item the only issue is mechusar amanah, an ethical flaw (which some authorities refer to as a prohibition; see Rosh, for instance) that will not apply to somebody who is unable to complete the purchase.
Therefore, under the circumstances there is certainly no obligation on children to make the transaction.