Were you able to address those reasons that led to you breaking up with her? Hopefully these are dealt with in order for you to move on without any unresolved past issues.
If the two of you have reunited and she's willing to give the relationship another chance, you can propose to her. See her reaction and you'd have a better idea about what she's feeling.
She may be frightened to rush into anything and you may let her know that you're willing to give her the time she needs. It can be both exciting and anxiety provoking to make major chances in life.
As long as you know that you love her and her child, you can let her do things on her own pace. The two of you can see how being engaged feels and what if any new challenges arise as a result of it.
How did she react to your letter?
I was going by what you shared about her reaction. She had indicated to you that she's open to trying again. You also mentioned that the issues that had caused the break up in the past are resolved on your end.
Proposing does not have to imply pressuring her. You could let her know that she can take her time hence, you're also giving her the chain necklace.
She is the one that ultimately would decide what she wants to do because of her free will and your respect for it. Meanwhile, you can still talk to her and see if she'd be willing to meet for lunch (it does not have to lead to anything intimate/sexual) just talking, seeing one another, catching up on what each one is doing. If she is receptive to that, then you could have few such meetings before you pop the question. Go by what she's showing you she's comfortable with.
If she is afraid of something, ask her if she'd be willing to talk to you about it. If not, you accept that too.
You can ask her to let you know what is the most stressful part of the idea of being together again, what would she like to see happen differently and whether or not she believes it's possible to truly fix things and start a new chapter. You can acknowledge one of her fears about being a stepfather or a male figure in her child's life.
In the end it is not really that proposing would break the deal. It is what she wants and believes is possible.
I think the space between us this past week and a half has been good. It has allowed me to clear my thoughts and I hope it has for you too.
The time and space has allowed me to reflect on everything and to make sure that I knew this is love. That this is worth every second and that the feelings are real.
Last week I made promises to you that I loved you. I made promises to you that I loved [your son] and I made promises to you that this, if you wanted it, is forever. I let you know that you are my dream and that I want to be with you forever.
Today will hopefully be a day that we both remember forever. Today I want to start bringing your dreams to life. You said that you dreamed that I would come back, and I did. You said that I was your dream man and hopefully today another of your dreams will come true and I will make you the happiest girl on the planet for now and forever.
You’re an amazing girl and I never want you or [your son] to leave my side, but I know that it’s time that I have to start showing you that not only have things changed with me, but that everything I have said is true. So..
(One knee) [Name], will you do me the honour of becoming my wife – Will you marry me?
(Followup of the answer) There is both a ring and a necklace here. This engagement ring is a symbol of my love for you. This necklace is a symbol that I will always be around for you, no matter what happens. I want it to be up to you which you wear. The ring when you are ready and the necklace if you feel hesitant. I want to give you the option of accepting, without feeling pressured all around.
I love you.
Isn't it what you want to do? You seem to have thought about this for a while now. The outcome would depend on how she feels. And, you're giving her options as well as letting her know of your intent. If she wants to consider reunification she can be open to this possibility. She's not completely broken the contact off. It may be good to see her a few times in person before you proceed w/ it just to see how she seems being around you.
She has free will to do as she wishes regardless of how you come across. This means, that even if you don't propose and wait some more, she could still decide to either reunite or called it quits because of what she wants in the end.