I take it you've not owned a trampoline before? Because making large structural changes to your garden (a massive hole is needed) is short sighted if you end up with children who are bored of it within a few months.
Also - if you do this, go big. A sunken trampoline is more permanent than a normal trampoline. It's usual for a pre-schooler to be fine with, say, an 8ft. But (if your children do stay interested long term - if) when you have a 10yo wanting four mates round to play, or a 15yo wanting to jump stunts, you're likely to want 14ft plus.
I have four children, only one of who has consistantly used the trampoline (two who use intermittently in the summer and one not interested). We've been through four trampolines in 13 years - 8ft, 10ft and two 14ft.
It's worth noting that the high frequency of families getting bored with trampolines means that they are fantastic value for money second hand. Indeed you can often pick them up free on Facebook because families can't be bothered to dismantle it themselves. They are actually very easy to dismantle and put up when you know what you're doing.
I mention this because it's worth spending up to £30 on a second hand normal trampoline for the summer, before investing in a more perminant sunken one. Just buy it for the summer and dismantle by autumn if you so wish. Then you can see how it is used and if the size is right etc.
We contemplated a sunken one in Lockdown 1 when we were redesigning a section of the garden. Decided against it in the end.