Mtoto ambaye anacheza nje anatoka Marekani sasa
Doesn't Sasa usually follow the verb it modifies?
yes, and the verb phrase it's modifying is 'playing outside', which is 'anacheza nje', as is given in the answer
Your sentence would mean that the child is coming from America now, not that they're playing outside now.
Here you modified anatoka Marekani
Why is 'Mtoto anayecheza nje sasa anatoka Marekani' not correct? Is this because the relative clause is at the start of the sentence?
This is absolutely the same. Ni sahihi kabisa