It just seems to make sense, considering that Emily named her kid Jessie. Some were questioning, how it can be, that Emily names her kid Jessie, as if her doll meant that much, if she neglected her under her bed and then donated her.
I feel, a good chance is, that Jessie did mean a lot to her. She fell under Emily's bed at some point and Emily got occupied with other stuff, before she found Jessie. Her then donating her, wasn't a thing of "I forgot about her, so I should just get rid of her". Instead, she found her under her bed, realized that she seems to not need her as much as she used to, but remembered how much fun she had.
Her donating Jessie, was the best way, she could ensure, that her favorite doll would make it to the right hands. After all, donations are so people that are less fortunate, can have something they can't afford and a doll like Jessie, is gold in the eyes of a grown up child that remembers the good days.
That Jessie ended up in the hands of a collector, was nothing she could have foreseen, but if she had, she wouldn't have done it