Must the poor who are fed as expiation for an oath be adults, or may some of them be children, given that the cost of feeding in Bangladesh is much lower than in the Netherlands?

Reaching puberty is not a condition for the poor person who is fed as part of an expiation for an oath. A poor child may be fed from the expiation food. The poor (masakin) are those who find some of their basic needs met. The amount of feeding for an oath's expiation is to feed ten poor people from the average food that one's family eats, such that they are satisfied. You are obligated to seek out the poor Muslims in your country for your expiations. It is permissible to transfer expiations to another country if there are those who are more in need there, and the difference in food prices between countries does not matter, because the obligation is to feed the poor or give them food, not its monetary value.