When using a front mounted child seat, will it be better to size up or size down on a Mountain bike (I'm right on the boundary between sizes)? I'm buying a mountain bike and come in exactly on the boundary between medium and large women's frame sizes. I used to commute by bike in a city several years ago so fairly confident cyclist but would now be cycling on trails with my toddler. I'm planning to use a front-mounted child seat - probably the Thule Yepp Mini. Has anyone used this seat or similar seats and can recommend whether it is better to go for the larger or smaller MTB size to allow space for my knees while cycling, but also be able to reach around the seat to the handlebars? Covid restrictions mean I can't try it out in advance. Thanks very much
Some things to consider (I'm certainly not an expert!):

Your toddler's weight, sitting high in the seat, will tend to make the bike top-heavy. Your legs are what will keep the bike from tipping over sideways, so a lower or slanted top tube (lower standover height) will make it easier to plant one or both feet on the ground.
Different bike models will have different seat-tube to steerer distances, which will affect your knee/Thule clearance as you mentioned. It might be worth looking at the men's models as well as women's models to find something you're happy with.
There are some reviews that mention the Thule limiting steerer movement to the sides on some bikes; maybe that's actually a good thing?
You or the bike shop can easily modify the bike, for example with a shorter/longer or differently angled handlebar, to suit your needs better.