Most definitely! Especially in the case of languages which have similar origin – there are words that are identical in some languages, making them easier to remember. Grammatical structures may also appear similar in some contexts, so when you're already familiar with your mother tongue (obviously

), you will need less effort to learn another language.
But we shall not forget about
false friends which, at first, look completely normal and valid, until you find out that other tongues use different phrases. There is a popular joke in Poland
– thank you in advance is translated as
dziękuję z góry. Some Poles, not familiar with with the English phrase, directly translated our Polish version as
thank you from the mountain. Such cases may cause a lot of confusion! So, as much as perfecting our mother tongue is a great help in the case of communication with foreigners, it can also end up with some funny situations.
As for the children, let's remember that small kids assimilate languages rather than learn them. It's a longer topic for discussion, but when a child is exposed to a bilingual environment from their early years, he or she will be able to master two languages. Because of the assimilation that happens in the child's critical period of life.