Is it obligatory to pay for sweets and food items taken from a vendor during elementary school 19 years ago, knowing that their price was negligible and not precisely remembered, and that one's age at the time did not exceed nine years?

The seller's right to the price of goods does not lapse due to prescription or the buyer's young age. The amounts must be returned to him with an estimation that most likely clears one's liability, as Ibn Al-Arabi said: "If he is confused about the amount of what is lawful and what is unlawful, then he estimates what he deems unlawful and takes precautions in that so that no doubt remains in his mind, and that his liability is cleared of the unlawful." If you are unable to reach him, you should donate the amounts on his behalf while guaranteeing them to him if it becomes possible to reach him later, as stated in Al-Rawd Al-Murbi': "And if its owner is unknown... he should donate it on his behalf with a guarantee."