I was at a small quiet beach last summer and a very large school group of what must have been reception age children turned up. At first I thought 'oh fuck' to be honest. But it was really lovely to watch them. They were adorable. It was obvious that most of them had never been to a beach. They were really funny about the sand - could hardly walk on it at first, and were wobbling around all over the place. a couple of kids were freaked out by it and had to be carried on by the teachers. They all quickly got the hang of it. I'm not sure if your child has been to a sandy beach or not but I felt so terrible that this was these children's first experience of it, that's really sad. That's a real disadvantaged life. It was lovely to see how astonished and delighted they were when they got used to it. It gave the whole trip such an atmosphere of amazement and exploration for ALL the children, even those who were used to the environment. They all enjoyed dipping a cautious toe into the sea as well.
The teachers set up a big gazebo thing for shade and they put down some mats. All the kids had hats on and little packs with water in. They were supervised in small groups and extremely well supervised all day. They were taken to the loo in small groups (2 adults). They were not allowed to get too hot and were sat down to have a little drink and rest regularly.
Apart from that they played, they dug holes, they did some beach cricket, they did a treasure hunt, they sat around having a sing-song, they ate their picnic, they collected shells and stones and seaweed to look at. they were safe and had a fantastic day. I'm sure they learned a lot as well as having fun.
Don't let your anxieties turn into your child's prison. I had an anxious mother who would stop me doing things everyone else was doing. Perfectly sensible things. Through emotional blackmail (she didn't see it that way, I love my Mum, she was great in most ways just depressed and anxious when I was little and it DID have an effect on me. I resented it). Your child knows everyone else is going. Your child will hear all the stories about it afterwards. Are you going to get your child to lie to the school and say they hadn't come because they were ill? I had that and believe me, I in my 5 year old way hated my mother for that. Are you going to say to your child, well ok those other children are allowed and the school has organised it, but it makes Mum too unhappy for you to go and won't we have MUCH more fun here just the two of us together having a lovely day. I had that. No, I didn't have MUCH more fun at home with Mum, I knew it, and I wanted to go and do the stuff with the school but felt unable to hurt my Mum's feelings. so I'd agree, yes I'd much rather sit at home with you, Mum, while thinking NO! THIS IS NOT FAIR!. And it wasn't fair on me at all. Putting your own emotional needs before your child's emotional and educational needs is a crappy thing to do.