You make a lot of amazing points. Children with time and explanation can understand a lot. However, I think it’s important to understand his father as well. It’s just as painful if not not more for him. Sometimes it’s also best to leave some things out especially for kids so small because tell them too much can burden them. Not to mention his father also hasn’t really resolved himself either and bring up things he’s unsure about can also confuse him. I’m just saying all this because you seem to have a one sided view. There is just no possible way to gauge how anyone especially I child may react to anything, so even in your approach it still could have gone badly.
I see what your saying. That's a good point too.
The adults in the child's life (most likely their parents) should always ask or include the child in the decisions that GREATLY affect their child's life. For example, divorce, pregnancy, new home, vacation/travels, etc. These transition in the child's life are huge transitions that can cause them stress. Even when going to grade 2 from grade 1 is stressful. (New teacher, new classmates new desk, new stuff, etc).
So, having a new mom is never Tora's business since FIRST OF ALL he's almost never there to be with her anyways. He's just marrying her to leave him with someone. So, please understand why he's not willing to move on from his mom. He was never given the choice to move on. It was like Tora just dropped a bomb where he gave him 5 seconds to run so that he can still have his organs intact but die from the blast or die holding the bomb. Either way in the end he dies. No option that satisfies Shiro. He should've mentioned that work changed and he needs to do more work, which means he can't bring Shiro with him anymore. Then give Shiro time to take that in and comprehend that and make him understand that there is no option for him to stay with dad. Next, mention his interest with school. Oh! You want to go to school?? Maybe you can go to the same school as your cousin's! Great, now Shiro is excited. Then mention the situation with Tora's sister. That he can't leave him to his sister cause she's having another baby. So another conversation where you talk about his options. Shiro, I can't leave you with anybody. I was thinking of marrying again so there's someone watching over you. Would you like that Shiro? Reassure him the truth, that you still love his mom but for the situation you both need a new mom to support you and Shiro. But this doesn't mean it perfect. But at least the child didn't meet his potential mom blindly. Respect Shiro's feelings. He just got over his mom's death and maybe still moving on. He's capable of understanding and comprehending his surroundings. Just cause he's a child doesn't mean he's dumb.