Make the riding ring larger -- at least 60 by 120, preferably more (see below for reasons) -- but leave it open for grazing when not being ridden in!!
That way you can have your cake and eat it too
Make the fence fairly high and strong and horse-proof -- 4-board would be good, if you can swing it -- to minimize horses playing bitey games with each other from opposite sides of the fence. Alternatively, instead of leaving it open constantly for them to wander in and out, open it for controlled grazing so that the horses are generally either all in or all out. Either way works fine and maximizes your grazing while still giving you a ring to ride in!
Two reasons why you will REGRET a small ring: first, it will not take very long at *all* for a path, then a rut, to get worn in the 'track' along the rail. Not only does this create bad footing (harmful to legs/feet, dangerous when muddy), it makes riding difficult and makes it difficult for the kid to learn to steer the horse independantly as the horse will tend to just plod round in the rut. With a larger wider ring, you can use cones or suchlike to define a ring *within* the fencing, and move it a yard or two to one side or the other every month or two, so that instead of constantly groovin' in a single rut the wear is distributed over a wider swath of grass. This can make a BIG difference.
Also, when your daughter gets old enough to be doing 'real' canter (as opposed to the sort of "world's slowest" type, done by oldsters and lesson horses and western pleasure types) even 60' width is awfull narrow and places considerable demands on the horse's balance and the quality of the footing. (Mud, or baked-dry ground, are very slippery, and I've seen horses fall over around corners a number of times over the years in such conditions when making tight turns...)
And finally, if your daughter ever gets interested in crossrails and jumping, even 65x130 (basically 'small dressage arena' size) is aaaaaawwwfully small. One can start out that way over crossrails if one has to, but it won't be long before the size of the ring seriously limits what she can do/learn.
Fences are expensive and aggravating enough to build that I would REALLY REALLY advocate making it comfortably big enough now. Enlarging it later is not really a good option to plan on.
Good luck, have fun,
Pat