We took our 9 month old in 2019 and had the best time! You kind of just need to accept that nothing will be in its usual time frame. Don't stress about naps and bed time. Baby will sleep when they're tired. You will miss some things and have a more chilled festival but you will do new things and make new memories bathing baby in the NCT tent and chilling in the kids areas. The Greenpeace kids is only for people with children so it's a quieter haven with a massive shady tree to escape the relentless sun.
We just let our little one sleep whenever and usually we went to bed around midnight with her. She actually slept the best she had ever slept as she was so exhausted from all that fresh air and stimulation and would be out for 5 hours straight.
You also will then wake up at 6am so you get to see the people coming home from last night while youre up getting coffee. As I said it's different but it's still amazing and we are taking her again next year when she will be 3.5 and bringing our new daughter who will be 20 months. We can't wait!
P.s I don't know if I can find it but if you Google baby at Glastonbury a few people have written brilliant, informative and assuring blogs about taking their babies, some only weeks old.