Japanese grammar, から vs. ので?

Both から and ので are more-or-less interchangeable in the context of meaning "so" or "because", and yes, ので sounds more polite than から.

There are a few subtle nuances where using ので instead of から (or vice-versa) would sound more natural, but those are more advanced topics that you probably don't need to get into right now. Like English, not every grammaticist agrees on what is considered "correct" grammar, and so some grammar adhere to a stricter set of rules as to when to use when. But like I said, you probably don't need to bother with those different interpretations until you're more advanced in your studies.

The most important distinction between the two that you should know, in my opinion, is just to know that から is more casual and ので more polite and formal. Therefore, you should use it in sentences that match the appropriate politeness level.

For example, you could say

こどもが いるから、まいにち たのしいよ

which sounds fine, but

こどもが いるので、まいにち たのしいよ

sounds a bit unnatural because the polite ので is being mixed with the casual よ.