Hi Funaek,
20k is the low end of the conventionally accepted MG range. So you may be okay if you can squeak past 20k, but it depends on a few factors.
Is this for the younger end of MG (7-9), standard MG ages (8-12) or upper MG (10-13)? The older the target audience, the higher the recommended word count range. Also, you said that this is fantasy, and fantasies generally have a higher word count; but there are again gradations here: is it a sweet little story set in our world with a single magical element, like unicorns or pixies? Or is it a fantasy set in a different world and/or with a complex fantasy system that requires a fair bit of world building? More world building means more words.
These rules aren't hard and fast. If you have a great story and your word count is low because you have tight, evocative writing, then you have a good chance of finding a home for it.
The only thing I'd say is that word counts that are very high or very low can trigger the auto reject for some agents, so you may deal with a statistical disadvantage if it's dramatically off from the recommended range. The closer to that recommended range you can get, the better your statistics, but ultimately what really matters is the story. A few thousand words shouldn't make or break you.
If you're still worried about it, you might consider posting a synopsis of your story in QLH when you've reached 50 posts to get some objective feedback on whether the story has enough meat to it. Even if you're not worried about, posting your query and synopsis in QLH is an excellent idea before you start querying.
Good luck, Funaek!