I saw the Rav say that granola bars are Ha’adoma, but for the after Brachah one should try to either eat it B’toch Seuda or to eat a small amount of it at a time, taking care not to eat a Kezayis or more within a time-span of Achilas Pras . My question is for children where the above two options are not possible, is it better not to give them this snack, or to give it and just tell them to make a Borei Nefoshos?
L’chatchilla one should not eat a Kezayis of a granola bar out of a meal. It is only B’dieved if one ate a Kezayis out of a meal that they recite Borei Nefashos.
Therefore, children should be educated the same way. You can choose one of these three options:
1. Don’t use this snack, as you proposed.
2. Use it as a snack only in a meal.
3. Use it as a snack, but only small amounts – less than a Kezayis – at a time.
About the Brachah Acharona on granola bars, see here.
About educating children in Hiddur Mitzvah, see the references here:
Our case is much more serious, as it involves the proper behavior L’chatchilla, not only Hiddur Mitzvah.