What's the regulation at immigration at Changi airport to have milk on hand carry (eg 100 ml pack required?) or frozen milk?

I don't think it's an immigration thing, more of an airline (LGA concern). According to this link from the Changi Airport website, if your baby is travelling with you, you may bring your expressed breast milk onboard the aircraft. However, if your baby is not travelling with you, it is strongly advised that you store the expressed breast milk into containers not exceeding 100ml. http://www.changiairport.com/en/passenger-guide/special-assistance.html