So a 10 year old is likely to be most interested in Saturn, Jupiter and the moon and some of the brighter star clusters. (Not much co.pares to your first look at Saturn)
These will generally (certainly the planets and moon) be fairly easy to find and not need goto. BUT what goto will give you is tracking, which keeps the object in the eyepiece for longer, allowing you to share the experience more with your son, rather than constant ‘no, its moved’ which could frustrate and loose your audience before they get started.
Bigger scopes don’t usually mean more magnificatiob, but make thinks brighter and more detailed
The heritage flextube will need something very solid to put it on,if on for example garden furniture, it will get knocked lots, or vibrations from you leaning on it will spoil the experience.
Equatorial mounts appear a bit complex and can out off some
To start with, a 127 or 130 goto reflector on an alt az mount would probably be easy enough to use, and give enough to see.
Like mountain biking, n plus 1 rule also applies when you want to take it a bit more seriously, and scopes also sell well second hand.
Agree with others that FLO are great, so are either valley optics and so is turn.left at Orion.
Enjoy