Rather than a funeral plan bought ahead, how about just setting aside enough money in a separate account for simple funeral, but leaving a letter or note in your will stating that you are happy to have a direct cremation if that what your children feel comfortable with.
I think a lot depends on circumstances and there's just no way of knowing how those closest will react.
My dad was declared 'end of life' with frailty early in the first lockdown. Mum had previously mentioned a direct cremation and I'd been horrified, but within the context of restrictions I could get on board with that and having a low key scattering of ashes with friends and family at a later date.
In the end he hung on for a year and by the time he passed away restrictions were being rolled back and we were able ro have a funeral and wake for 30 people which was fine for us.
The period in between his death and funeral was nearly 4 weeks, which seemed like a long time, but it meant we didn't have to rush anything, and those weeks gave me the chance to look back over his life, my memories and photos of him.
The input I had into his service (mixed as he wasn't religious but we did include his favourite hymn and a prayer for mum's benefit) was really meaningful for me and I'm proud of the send off we gave him and still comforted by it months later.
People's reactions are so difficult to judge...mum, who is normally squeamish, visited Dad at the undertakers with an open coffin 4 times. I'm normally quite stoic but I couldn't make it fully into the room and had a closed coffin visit the day before the funeral.
Even now, mum keeps talking about what we're going to do with his ashes and I'm just ready to make that decisions yet...again I'd have expected that to be the opposite way round.
What I'm trying to say amid all this ramble is I think giving your kids options when the time comes is the kindest way to go about it.