I don`t think Simba and Nala had a crush on each other when they were cubs.
In my opinion, Nala is glancing at Simba because she is feeling sympathetic towards him. Simba clearly feels bad for disappointing his father and is aware that he`s in deep trouble. Nala tried to make Simba feel better by telling him how brave she thought Simba was.. he did try to protect her and even saved her life!
When the cubs are walking together, I think they both feel ashamed and worried about the reactions of their parents, but I think Nala mostly feels bad for Simba and wants to cheer him up or comfort him - she looks at him, but notices how he keeps his gaze down and doesn`t speak, so she also doesn`t know what to say...
Hope my post makes sense.