Probiotics.thinking of taking them to boost our immune system. are there any negatives to taking them and is it safe for my 4 yr old child?

Probably not: Unless someone is dangerously allergic (anaphylaction), no. Since a lot of probiotics are from bacteria that occur in milk (such as lactobaccillus), theoretically, if someone is anaphylactic to milk, an allergic reaction is a possibility.

Probiotics safe: Many people are looking for some "magic pill" to "boost their immune system". There is no such pill. However, there is a way to boost your immune system, live a healthy lifestyle. Proper diet, proper sleep- wake cycles, healthy environment, proper exercise are the foundation of good health and good immune health. Probiotics should be used after antibiotics to replace "good" bacteria in your gut.