I wish I knew more about what you mean by "too much for her"? Each child is different but in general, it's best not to keep secrets from them; the resentment and lack of trust that builds up once the truth comes out -- and it always does, sooner or later, often in ways you had never anticipated -- can poison a parent/child relationship forever. If the fact of your relationship with her father is well-established, I'm not sure of the benefit of waiting to tell her. This is the kind of issue where a therapist can be helpful.
Her reaction could run the gamut, from being thrilled to having a baby in the family and wanting to babysit, to being angry at him for "doing this to mom," whatever that might mean to a 10-year old, to just being plain angry. Other thoughts:
1. The longer dad waits to tell her -- especially years, not months -- the more the potential for anger and resentment.
2. When he does tell her, this should be between the two of them. You have no role in the initial conversation.
3. He should keep the conversation simple: "After your mom and I split, I met a woman named X. We had a baby and that baby's name is Y and he's 6 months old. You'll probably have a lot of questions after you think about this. You can ask me anything."
4. One of her first questions will likely be, "Does mom know?" This is important: mom needs to know before the daughter does. The daughter should not be expected to keep this from her mom.
6. Let meeting the baby be the daughter's choice.
5. Here's a question that typically does not get verbalized: "What about me? Do you still love me? Where do I fit into this?" This question needs to get answered even if it's not asked. "Of course, I still love you. You're still my daughter, you will always be my daughter and I will always love you." Another likely question: Will you have more babies with Alayna?" Answer that truthfully but also as if she has asked the "what about me"
6. Wait to see what her questions are before offering more information. Likely questions: Where does he live? When do you see him? If this is really complicated -- for instance, the mother doesn't want him involved -- that explanation needs to be age-appropriate but also truthful: "The mom doesn't want me involved. Maybe some day, I will be, but right now I have to respect her wishes."
7. If she asks you a question, a good answer is, "I'm not sure. We can ask your dad together."