Super light weight aluminum kids racuqets are not intended for high paced yellow/regulation balls and really should not be used. If a child is using yellow balls they should use a heavier, longer 26"+ racquets to match the weight and pace of said balls. When I talk about "pace" I am referring to "rotating mass at high speed." Meaning, the faster weight travels with rotation the heavier it impacts a racquet. Pace with yellow/regulation balls can completely overwhelm light weight racquets.
If a child cannot handle using such racquets then they should be on progressive courts with lighter, slower paced balls and smaller racquets.
Soapbox:
I do not think 5 year-olds should be on the fullsize court unless they can play the game on that court size. If they can then great! Tennis development isn't a race, it's a marathon. Yes. I went to the cheese.
If they cannot serve, groundstroke, transition and volley on that court then they are not playing the game on that court. Otherwise they should at lest be on the 60ft Court, the second progression of court sizes. The first being 36ft and the last being the fullsize 78ft court.
To first train a young child on the fullsize, 78ft court creates what I call "Tupperware Tennis Development." This locks a child in a sterile environment where every ball is feed perfect; and where said child has to wait a prolonged time to actually play the game. The point of playing Tennis is to play Tennis.
I agree with your first 2 paragraphs.
But I do not agree with the last part. The worst thing you can do is feel the need for a kid to 'play tennis' to soon. Rick Macci, Sanchez Vicario and other top coaches design proper drills that push kids on a full court, they never just give perfect feeds.
When a kid starts to 'play tennis' right away on a tiny court they barely sweat, barely move. They can cover the court with a few steps. These kids are shocked when they later realize how much running is involved in real tennis. The small courts and playing tennis right away creates the Tupperwear Tennis you mentioned.
There is no easy way. You want a future rec player let them play Quickstart right away and play points on a tiny court. Let them think tennis is not a running sport. But if you want an elite player use the Macci or Sanchez Vicario way of hand feeds and racquet feds, pushing them to move around a full court, fun and challenging drills for years before they every play 'real' tennis.
The future elites do everything you say, transition, volley, serve....but they do it at young ages in the context of drills and feeds that push them to the edge of current abilities. They do not do it on a tiny court with slow balls in the context of playing "real tennis".
It is what it is, no short cuts to producing top players. Ask any top kids coach and they will all tell you that the earlier a kid is allowed to play 'real' tennis on a mini court, in the vast majority of cases, the worse their fundamentals will be, the worse their hustle will be, and the worse their overall success in tennis will be.
Tennis is the only kids sport they call their own lines, the only game they have no coach or caddy with them during play, the only physical game that they have no teammate to come in when they get tired. It is a unique sport and to develop a top junior they must be developed unlike any other sport, and that includes no early 'real play'. There is simply no way for a 5 year old to actually play real tennis because mini courts are not the same. In bitty basketball they still get banged and get tired, in mini baseball they still have to race to the bases, in mini soccer they still run a lot. But in almost all mini tennis they move 2-3 steps and tap a ball and can use awful technique to have early success....its just not tennis and is not useful for long term development. Those amazing kid's mini tennis points you see on You Tube are very rare. But it is just fine if you want hack weekend players down the line at the local park....nothing wrong with it if that is your goal.