Well, you still have vaginal walls. And they likely have not increased in size.
However, two things can happen after childbirth which can effect sensation during intercourse or other vaginal penetration.
The muscles of the vaginal floor can loosen a bit: tightening those up is acheived over time with what are called Kegel exercises. A kegel is done simply by squeezing those muscles together, just the same way you squeeze them -- whether you knew what you were doing or not -- after urination to squeeze out the last few drops of urine.
As well, the experience of childbirth, as well as some hormonal shifts, can cause differences in how you experience this sensation. Without getting crass, if you're a size queen of sorts, then yes, after childbirth, a mere penis isn't going to be all that impressive.
Do the Kegels, a few times a day for a few minutes at a time. You can do them anytime you remember, wherever you happen to be because no one can tell. So, waiting in line, standing in an elevator, making dinner, taking a shower...et cetera. (You can also do them during intercourse or other penetration.) Also, getting a bit more active again is often a help, as that ups overall muscle tone in your body and increases testosterone levels, which can increase general desire.
Lastly, intercourse alone tends to get boring for most people after a certain time, no matter what. It doesn't tend to stimulate the entire vulva very well: in many ways, intercourse can be better as an appetizwer or dessert than as a main course. So, using this time to explore other sexual activities with your partner now is something else I'd suggest, like manual sex or oral sex for the both of you, like anal play for either if that appeals, mutual masturbation, role play, and so forth.