So I was driving and I accidentally put the transmission into park, I was doing maybe 4 or 5 km/h I didn't hear anything unusual other than the tires squealing, drove fine after that but I was wondering if I should take it to a mechanic?
Shifting into park engages the parking pawl inside the transmission and physically locks up the gears. If that mechanism was engaged at sufficient speed and no safety existed to prevent it, most likely the pawl would be wrecked, although other components could also fail. In any case, catastrophic damage would have manifested symptoms right away, probably with problems shifting in or out of park.
If you haven't seen any issues driving it, then the car is probably OK. It's possible that some sub-critical damage was done that will ultimately shorten the life of the transmission, but even if that were confirmed, having it fixed would be prohibitively expensive.
I'd just keep driving it any be extra mindful to not do the same thing again.
Edit to add:
Transmissions are designed so that the pawl will ratchet across the parking gear if the gear has sufficient rotational energy. If that weren't the case, then accidentally shifting to park on the freeway would be incredibly dangerous; even if you managed to not die or kill someone else, the car would be destroyed.
But at the same time it needs be nearly impossible to disengage the pawl when the car is parked. As kids, did you and your friends ever try rocking a parked car? It probably didn't work, because the backlash in the parking gear is too small to allow enough momentum to be generated to overcome the spring force that holds the pawl in place. This is mandated by federal vehicle safety requirements.
So there are 2 competing needs: the pawl must slip at sufficiently high speeds, but at the same time must not slip at all when stopped. This results in a gray area in which the car is moving, but not fast enough to keep the pawl from locking. Quoting the comment from @rooby:
[...] the energy of the engine is absorbed by various parts of the drivetrain, which might flex, compress, bend, etc. The difference between destroying or not destroying the gearbox is how much energy those parts have to absorb. In this case the speed was not enough to break things (although might have weakened things during that stretching/bending/compressing of parts).
Vehicle designers know that people are going to make this mistake, and they do a good job of building cars that are forgiving (or at least will isolate the damage to a specific and less costly component, ex: shear pins on a snow blower). As long as it still works correctly, there probably isn't any catastrophic damage. Just remember to stop with the brakes next time instead of the transmission.
While it's definitely not good to do this, if it's running/driving fine, you've most likely suffered no damage. Considering the speed at which you did it was so low wasn't very fast and shouldn't have put a lot of stress on the drivetrain. If there was going to be a problem it would most likely have manifested right away.
If indeed damage was done, you'll know it soon enough. What's a mechanic going to do? Disassemble the transmission and look for damage? That's going to cost about as much as a rebuild, if that's even possible.
Many CVTs are not repairable in the field anyway and the manufacturers just sell the entire transmission as an FRU.
I'd just let it go and not worry about it. Chances are that there is no damage done and it will be fine.