What is the criteria for entry approval in Singapore??

The criteria are not published and likely to change frequently, due to the pandemic situation.
There are only few reports of actual experiences on this forum, from which we could deduce the criteria.
I think it is most likely (in meritocratic Singapore) that they are looking at the benefit an applicant would give to the country once admitted. Thus people working in essential sectors have high priority, dependants with babies not. For sure, the wishes and personal situation of you and your baby do not feature in this consideration.
You and your husband basically have to make the decision whether to wait (for an indefinite time) or he comes back to your country.

There are many people who are sailing in the similar boats as of yours. Enjoy and spent time with your dear ones until the situation stabilized. I am positive, in next couple of weeks, situation may improve and you can travel. Just wait for the good time and keep applying.

Of course, there is no streamlined criteria documented but priorities will be given based on what impact you will make to the country. My friend recently  got admitted in first attempt as WP holder(existing) and he was given 60 days of time to enter. Of course, he is in biotech industry.

Thank you for all your kind words...hope situation get back to normal asap..