I don’t for a minute believe that God sat down and decided to create childhood cancers, out of some malevolent wish to inflict suffering.
I think he created the universe, in the moment we refer to as the Big Bang, and allowed it to evolve freely.
Among all the myriad life forms that came from that act of creation were some that humans wish hadn’t- such as parasites, viruses, bacteria. And as our cells need to keep reproducing and repairing, sometimes the DNA makes a mistake and we get mutations, some of which result in cancer. We also contribute to the cancer toll by our own actions, such as smoking, using asbestos, using carcinogenic chemicals, etc. People also age, have accidents, get ill, get involved in wars - life does not come with a guarantee of an easy ride or longevity and prosperity.
But the fact is, life is temporary- we all have to die. And from God’s viewpoint, in eternity, a life of five years or a hundred and five years are both just a blink of an eye. The bit that matters is what comes after it - freed from time and space, as souls in communion with God’s love.
My own husband died young, at 36. He didn’t get to see our baby’s first steps or hear her first words. And I have grieved him for 31 years. You might say why did God allow that, and expect me to be angry with Him. But God didn’t cause DH’s brain haemorrhage, it was just bad luck.
I fully expect to be reunited with DH when I die, and look forward to seeing him again. His granny, who was widowed for 50 years, once said “I’m not a very good Christian - I don’t much care about meeting God. But I do so want to see my husband again!” I understand exactly how she felt, but it is God who has kept me going and supported me through three decades of widowhood, and I do care about meeting Him, humbling and awe inspiring though that will be.
Sorry, I have rambled a bit off topic, OP, but these are huge existential questions that you are raising, and always interesting to ponder!