My DS is 4 so I haven't had to have this conversation with him yet. But I have watched my siblings navigate these conversation with my niece and nephews, who are a similar age.
Lost my DB to suicide 11 years ago.
First of all it's impossible for the kids not to ask questions about this- I speak from personal experience. My brother died shortly before my niece and nephew were born, so they never met him- but despite this they're still very aware of his life. There are countless photos in my parents house, and of course there's an 'air of sadness' (to quote PP) when his name is mentioned, despite us all doing our best to put on brave faces.
To take his photos down, or to hide bereavement (and not display 'an air of sadness') would be to model pushing away feelings- which arguably would also not be a healthy example of how to handle feelings.
I was present when the conversation was had with my niece- and it was very much led by her. It was just myself, my sister (her mum) and my niece. It wasn't at all pre planned- it just happened and was led by her.
She asked very proactively what he was like. We spoke about him honestly - that he was amazing, and would have loved her very much.
She asked how he died- we told her very gently that he was feeling very sad, and made a decision that he didn't want to be alive anymore. And that ultimately it's really important to know that in life if you ever feel sad, you should always talk to someone, because things can and will always get better with love and support. And likewise, if a friend ever feels sad, always ask them if they're ok.
But also be prepared for some difficult questions too, I was asked a couple of really difficult questions directly which I wasn't at all prepared for.
She asked me why I didn't call him that day to ask if he was ok. (Hard to hear!). I just said that I just didn't know that he was feeling so sad, but I wish that I had called him, and sometimes in life we do things that we can't take back. But then reinforced that it's really important that she just knows that we all love her very much, and that she can always talk to either of us (I meant her mum really, but of course me too!).
And I also told her some funny stories about him to make her laugh- suicide is such a big traumatic event that often it over powers the memory of someone's life. And we both felt it was really important that her imagination think of happy, funny stories when she imagines hun (she's curious, so of course she will), rather than the dark vision of suicide.