Hi Yes, there should be a decent amount of water in the Merced River in July. You can't walk to the base of Vernal Fall, but you can go up the paved trail to the footbridge which has a view. Or you can see the waterfall from Glacier Point. I highly recommend using the Yosemite forum instead of this seldom seen Q&A feature. Let folks on the forum know if you're using Google Translate when you post in English on that forum.

You can get a good view of the falls before the steep stairs section, but you need to go well past the bridge. The paved path leading to the bridge is a pretty steep incline, then you need to go another half a mile or so which is not as steep and follows along and overlooking the river, which is very inspiring and you will make many stops to take pictures. Then you will get to a point where you can see the falls, just before the steep incline. You may even be motivated to take a few sections of the steep stairs to get a little better view, it is so awesome!

Vernal Fall is fed by the Merced River. It always has water, regardless of the snowpack or season. On average, the highest water level is usually May/June and the lowest water level is October. Yosemite Falls is fed by seasonal stream Yosemite Creek and will usually stop flowing in late summer or early fall. June / July is fine for water flow.

With my 9 year old and 6 year old, it took us a little over 4 hours. We also had 20 min at the top to enjoy the pool of water. We probably walked a slightly faster than average pace based on the amount of people that we passed.

Il percorso è in salita, su un bel sentiero largo e nella parte bassa asfaltato. Un po prima della cascata si stringe e passa tra pietre e rocce, ma nulla di vertiginoso. Sono circa 3 km per andare e 300 m di dislivello. Se il bimbo di 4 anni cammina non penso ci siano problemi, nell'ultima parte lo dovrà tenere per mano, anche perchè genere c'è anche parecchia gente ed il sentierino è stretto. Se decide di andare in cima alla cascata, sulla destra c'è una scalinata tagliata nella roccia, con scalini irregolari, magari lo dovrà aiutare ma sopra c'è un bel laghetto. saluti.

No. You need to be further from civilization in Yosemite Valley. Check the NPS website about backpacking.