Seriously what kinda husband suggested that? There a reason why they invented baby toothpaste... Reason being adult toothpaste is more stronger and some toddler doesnt know how to spit out yet and may tend to swallow the paste. The cheapest toothpaste is only 1-2$ and can last for a month.
2ndly, if that makes ur mil happy, den let her be. But make sure she dont go overboard. speak ur mind out. Ur the mother and u know what best for ur child. If ur mil want ur child to be the best fren, let her try. Think of it as, ur child have a fren that can always play so he doesnt feel lonely. Cheers mummy...