Heart Wrenching Question

So Zarah started school this year, we were super excited that the day has finally arrived. A day that we prayed and longed for, for many years. But we werent quite ready for the heart wrenching question that would eventually follow. She has now became aware that her peers in her class either have older or younger siblings. I was totally caught off guard when she posed the question of when she will be getting her sibling, why she doesnt have living siblings (yes she is fully aware of Zachary and Zoe in heaven). I tried explaining but our infertility and preganncy journey is so complexed for her to understand, that I eventually just answered with a blunt we will try. She looked all confused but I left it at that hoping the questions would subside. But as more and more friends and family now start having their second child the more she started probing. As a mother I almost feel so guity at times that she is the only child, I try to remind myself of a colleague of mine who is the only child saying "I was the only child and turned out fine". I continue to pray that God will guide us and lead us into the right words so that we are able to explain to her in an simplistic way one day. I thought I was ok, but until the questions started and of late I have been yearning out of the blue for another child. But after tryng 2 years agao on two occassions, we have made the promise to ourselves that we are not going through the emotional rollercoaster again. This time we have to think of the possible implication that it will now have on Zarah too. It's emotionally draining and we need to be mindful that we have young child that will feed off the impact of this journey too.