Grief of a baby help

I feel like I post too much but this is my only safe place. I have custody of my 5 nephews. 12, 7, 5, 2, and 2 months. October of 2017 one of my nephews passed at 9 months old from AVM a malformed blood vessel in his brain that burst. I still struggle badly with it. I thought the kids were doing ok and I got a cake and we sung happy birthday to him on Christmas which is his birthday. I thought I was doing everything I should. Until this morning after the ball dropped and my 7 year old was talking about dollars, he said he didn't money to which I replied yea bc you have everything you want. He says except one thing and I say oh what's that? He points up and says Maverick here with me, I was shocked. I said I can't buy that with dollars and I said besides if he was here you guys would be even worse as I giggled bc they can be rowdy alot. I turned and went to my room bc I couldn't hold in the tears, I had no idea he and the 5 year old followed. I was bawling and my husband was freaking out thinking something bad happened. The 7 year old says oh....aunt Nikki is crying and motions for the 5 year old to follow him. When I finally get done crying hard i tell my husband. It hurts. I'm glad they talk about him but it took me by surprise and what do I say to a 7 year old that wants his brother back? I can't even talk to the kids or anyone really about maverick bc I cry. I just need advice on how to handle and talk about this to small children. I never realized they even misses him bc I'm always caught up in how I feel and I feel so horrible. Then I couldn't even say anything right when he said that.

These pictures of my sweet 7 year old losing his brother. He took it hard then but they never mention him really so I've not thought about it hurting them. Someone please give me advice, please