As stated before this is more of an interpersonal relationship issue then a parenting one, which I'm not at all qualified at tackling (I'm aromantic, so if I tried to advice you there it would sort of be the blind leading the blind :P )
However, I can speak to the concern about meeting other children, I have seen a similar situation, different details but the same idea of 'I'm not sure kids I want to introduce kids to this person or concept yet' mindset many times over as a side effect of my volunteering.
I can tell you three things things that are important to know. First the kids can totally handle it. Kids are quite capable of accepting things like this, in fact for the majority of children they will accept it without even asking any questions. This is less about them fully understanding everything and more about their just not caring. Kids care about who spends time with them and cares for them, they care about who they play and have fun with and what their daily lifestyle will be like. To them concepts like 'complicated' adult interactions are too complex to worry about. All they care is "will this new women I'm introduced to be a fun playmate or another boring adult for me to ignore?" They may be mildly interested in meeting a toddler, but not due to their biological relationship, just because having a new toddler playmate can be fun for some kids regardless of how they are related to the toddler.
So I believe the 6 year old will accept things without any question at all. The 8 year old is a bit older and may ask some questions, but they likely will not be overly concerned. Stress the basic concept that Daddy loves this women, but still loves them and now they just have even more people that care about them (assuming you actually want to be a part of their life in the long term) and they will accept it. It won't be nearly as disastrous or hard as your husband believes.
Fact 2, the longer you wait the harder it gets! There are two reasons for this. First as kids get older they get more curious about adult relationships and how it affects them. As I said already I anticipate the 6 year old just accepting things with the 8 year old more likely to ask questions due to their age. The older the two kids get the more likely the are to start asking the more complicated questions. Thus the sooner they are introduced the easier the introduction. If they meet you now then by the time they are old enough to think of the more complicated, and awkward to answer, questions their relationship to you, and your child, will already be normal to them so they will just grow into better understanding how their related to you two naturally without any shock or difficult conversations.
The other reason it's better to tell children when they are young is so they feel you are honest with them. The longer you go without introducing the kids the longer you are keeping you and your child a 'secret' from them. I don't know if this is an actual secret, that the kids don't even know you exist, or if the children know of your existence in an abstract sense despite not meeting them, but the concept is the same. by not introducing them he is implying he wants to keep something from them. At their age their young enough they won't pick up on that, but the longer you wait the more likely you are to get questions about why it took so long for you to be introduced to them, and was his refusal to introduce you because there was something 'bad' about you or your children, or something 'wrong' with them etc. It's just an increase risk that they feel he was being deliberately secretive and not honest with them.
Fact 3, neither of the other facts will stop parents from feeling afraid to introduce kids. I've told all this information to many women. I've pointed them to child psychologists and social workers who all recommended doing these sort of introductions early (the folks I cite are speaking specifically to my volunteer situation, though I suspect the same advice would apply to yours as well). Despite this plenty of parents have been apprehensive about introducing children to other's in their life or being open about sharing details they thought were 'too complicated' for their kids.
In truth I don't think the issue is that the information is too complicated so much as that the parent is afraid. They are anticipating a hard conversation with lots of difficulty explaining things and that sounds soo difficult it's tempting to put it off. Unfortunately putting it off only makes it worse, it really is better to tell children stuff early before they have the wherewithal to question things so they will accept things without any of the difficult questions you want to avoid having to answer.
So in short I don't think your boyfriend is correct in his belief that they are too young. Despite that fact this sort of fear isn't entirely unheard of in parents. Unfortunately just pointing to facts, or advice from some random man posting on a stack exchange site, no matter how stunningly handsome and insightful that man may be, rarely helps to alleviate that concern.
As to how to convince your boyfriend that it really is best to do the introduction now, I'm afraid I can't help you there. My expertise with humans stops the moment they become teens, at which point my best advice is lock them into a cave until they turn 21 :P. There is an interpersonal stack exchange site that may be best for asking advice if you want to know how to convince your boyfriend to allow you to meet the other kids.