For a 20-month-old, BioGaia is quite well-regarded among mums I know, my friend brought it up when her little one had tummy issues, and she said the Protectis Powder or the drops are very gentle and easy to give to toddlers that age, clinically researched from newborn stage too. BiO-LiFE AB Junior Pre & Pro is another one that comes up often, it even won a Best Children's Probiotics award, and it combines both prebiotics and probiotics which some mums feel is more complete. Some parents are happy with either one, though a few mentioned price can add up, so it really depends on your budget and your child's tolerance. Do check with your paediatrician first before starting, as they can advise which strain suits your little one best, semoga berjaya, Mommy!