Good question. Use of 'will' implies a fact, whereas 'going to' implies consequences, e.g. 'if they eat all those sweets, they are not going to feel well'. A word of warning, DL is crap at making the distinction, hence why native English speakers have problems with it here.
sentirsi and sentire have different meanings.
1) to hear
Lo senti? = Can you hear it?
Stammi a sentire (listen to me!)
2) to feel (with physical feelings)
sento freddo (I am cold)
sento un dolore alla gamba
3) to smell
- sento un odore di gas (I smell gas)
4) followed by a noun it has the same meaning as provare:
- sentire paura/ provare paura (to feel afraid)
5) sentire qualcuno
- to hear from somebody
6) expresses that you have an impression (sense)
- sento che sta per succedere qualcosa
sentirsi is only to feel
I would understand: Non sentiranno bene as They are not going to hear well.