'If only' has a similar meaning to 'I wish'. The past tense is use as a distancing-in-reality tense. Grown-up people cannot see with a baby's eyes; we are dealing with a counterfactual hypothesis.
If I have time tomorrow, I will help you - possible.
If I had time now, I would help you. - hypothetical; I do not have time.
I wish I had time now. - hypothetical; I do not have time.
I wish grown-up people saw with baby's eyes. - hypthetical; they don't
If only grown-up people saw with baby's eyes. - hypothetical, they don't.
- For Teachers