Field trips
You may write about your child's reactions after a field trip. You may of course speak to the teachers directly instead of writing in the renrakucho.
You may choose whether or not you wish to write about your child's attitude before the event. Since the field trip is a school event, you are also not obliged to thank the school for the event. However, if you feel that the teachers really made an effort on behalf of your child, you may of course choose to express your thanks.
Model
Conversational flow and useful expressions
Greeting
Child's attitude, etc. (before the event)
Child's attitude, etc. (after the event)
Thanking the school (optional)
Parent's name
Supplement
「お世話様でした」(osewa sama deshita), which conveys a sense of indebtedness and gratitude, is an expression used to say "thank you". When「お世話になりました」(osewa ni narimashita) seems overly polite and seems to create a sense of distance between you and the other person, "osewa sama deshita" may be used instead.