Young children and chair seat

[SeventeenTwoChestnutMare]

During lessons, DS (nearly 5) looks to be in something of a chair seat, which seems to make rising trot quite hard work (and I worry a bit hard on the pony when he sits heavily).

Is this inevitable when a small rider, with necessarily short stirrups, is in a saddle that can accommodate larger riders?

Asking here because I don’t want to ask the instructor from a complete know-nothing position.

It’s quite difficult for them to find their balance at that age.

How big and, more importantly, how wide is the pony? Ideally he needs a very slim pony so he can use his inner thigh muscles to control his rise and fall. If the saddle is too big that certainly won’t help.

It is very common for DCs to start trying for a rising trot from the first lesson, I know everyone does it like that, but IMO they are better off finding their balance in sitting trot on a very well balanced pony. That kind of pony is really hard to find though.