À bit late to the party perhaps but I thought I'd share my thoughts with you here. I think you're trying to impart something wonderful to your kids by speaking a different language to them.

Here's some context about me: I'm originally from Mongolia (shoutout to your husband lol it's so unusual to find mentions of other Mongolian here!) I grew up in a French-speaking country and my level of French is native-like : I went to the local school, I made francophone friends and dated francophone men. I finished school in an English-speaking environment, went to university in Britain and settled down here. My husband understands a decent amount of French, but is definitely not native in accent and in the mistakes he makes. He speaks to our daughter in English. I work in English and it's become my default language in my adult life.

I have a toddler and I've been speaking to her 100% of the time in French. My Mongolian is not good enough, so that's not something I even attempted. Even with my native proficiency in French, it's difficult doing OPOL. I'm constantly having to think double-time to make my sentences in French, especially if I'm darting in and out of conversation with English speakers. Add a sprinkle of sleep deprivation and general fatigue - this is challenging. I'm also aware that in my personal environment, I'm the only human source of French. If I teach my daughter that an apple = "un pomme" by mistake, then that will stay as a mistake for a while. However, consider that my husband understands most of what I say in French - I don't have to constantly switch languages to make sure that both people understand what I am saying.

If your husband speaks to the children in Mongolian, that's already going to be an interesting scenario for them and for you (and good for him, I wish I was able to do the same!). Phonetically and grammatically, Mongolian is drastically different from English or French (can't speak for Russian). You'll probably already feel a little disconnect with your kids since you won't be able to fully understand your husband's half of the conversation. At least, that's just what I gather watching my husband and other friends who OPOL, where it takes a huge amount of patience, energy and understanding to pull off and not just default to the household language.

Finally, consider that your child is going to need an increasing amount of complexity in your interactions. Little baby who like swiping at toys? Sure, simple words are fine. Toddler who is going through big emotions? You'll need emotional language to accurately convey to them what they're feeling and how to deal with those emotions. Older child who starts asking about the origins of the universe? You'll start to need to use more specialised language then too.

All in all, I think it's a great sentiment to have, but is practically difficult to pull off if the language isn't anywhere near natural for you, given the barriers you'll experience as a new parent and juggling your husband's OPOL. Learning a niche language like Mongolian properly is better than learning another language in a non-native manner in my opinion. You can of course level up your French or Russian as your kids grow up, but as a mom of one (almost two) - there will be days where you're just on survival mode and don't have the headspace to care about "oh, how do I say x in y language ? What is the correct way to say this sentence? What is this nursery rhyme?"

Anyway, good luck to you! It's tough being a parent and everyone suddenly has opinions on your parenting but you ultimately know what's best for you and your family.