"Generally healthy happy 4.5 yr old grandson. Eats wide range of foods, including vegetables (broccoli a favourite) some reaction to dairy products so use soya and almond milk. Quite tight control on junk food, few between meal sugary snacks etc."
Hi, you are a lovely grandpa! I will start from the premise that he is healthy but underweight, and at the same time, ONLY dairy is out. I am also assuming that family diet is very healthy and therefore likely low-fat. I read the issue at hand as possibly (1) too little calories (2) eating slowly/ not really eating despite availability of food.
(1) Both my boys were dairy-intolerant in their early years and my mum is a very healthy vegan who has been cooking low fat for years. One thing I realized very quickly was that it takes effort to put enough calories into little kids without having dairy on standby. Consider that other kids are either guzzling full-fat milk or eating cheese as a very dense form of calorie intake, we have to really make an effort to make up for the loss in calories. Little tummies can't stretch much, and if he has been eating little for a long time, he will feel uncomfortable with a greater quantity. This is physiology. To get around this, you have to give high calorie food in small but frequent quantities.
My kids were not on a low-fat diet, but they were on a healthy fats diet - olive oil, flaxseed oil, coconut oil, sesame oil, avocados, salmon, dairy free-spreads. These were incorporated into their meals to up their calorie intake. I gave them a lot of potatoes as well in the form of mashed potatoes, in beef stews and as baked wedges because I read that potatoes were the best starch for putting on weight.
My boys loved vegetables and fruits, but I had to make sure that they were not taking so much of that that they could not fit in their carbs and proteins.
If your grandchild is taking low-fat snacks (eg vegetable sticks etc), the caregiver can perhaps add a high-calorie dip (hummus) or switch it up sometimes with something more calorie dense such as muffins or granola bars.
(2) There are perfectly sound reasons why a child may not be eating despite availability of food. One is allergic rhinitis which affects the sense of smell, which in turn suppresses appetite. Signs of this are runny nose in the morning, puffy eyes, possible snoring at night. Another is zinc deficiency which also affects the appetite. If he has not been eating enough for a period of time (eating enough as opposed to wide variety), it may be a good idea to add a multi-vitamin with zinc.
For actual mealtimes, I will go with three guidelines - tasty, visually attractive, easy. Tasty to him, attractive to him, easy to him. Both my children are on a fairly healthy diet but they still have very distinct preferences when it comes to textures and certain smells. E.g ds1 loves thigh meat and raw salad, ds2 eats only breast meat and prefers cooked vegetables. So you may want to take note of the flavours and textures that your grandson enjoys and come up with a list of corresponding dishes. Just make sure that there is at least one meal that he does like everyday.
Since he is very imaginative, the caregiver can perhaps create theme plates around favourite movies/shows e.g http://www.boredpanda.com/creative-bento-food-art-samantha-lee/ We can't all spend so much time making these, but perhaps the images can provide some ideas.