My eldest turned 6 the day she went to her first DL. I would suggest you bring a picture bible, and if you can bring some icon cards for her to shuffle thru and look at that is good. If you can let her pick some icons online and then print and laminate them that is ideal. It can be kind of boring for a 6 year old at first. They don't know what is going on and it is really confusing. My then 2 year old and 4mth old adapted more quickly than my 6 year old. If she can read, I would show her the text of the DL to follow along with. In any case-explain what is and is not expected of her well in advance. There are certain points where she MUST stand, otherwise she can stand or sit at other parts. Set up a hand signal that she can use if she needs to go potty or leave the service.
This is great advice from a woman at my parish-explain the length of the service as the length of your arm and when your child asks how much longer it is show them by the length of your arm left.
When we first started going we went to Orthos as well as the DL. The parish church school was during Orthos, so we had the 2 year old and 4mth old with us at this time. So we would be at the parish from 8:15 until after coffee hour was over at 1:30. The coffee hour (lunch) and all the playtime she could have with the other kids is what sealed the deal for my kids. There is a big yard around our parish for the kids to play in and my daughter loved having the freedom to go off and play right after eating.
Almost a year later my eldest cries and is really upset when we don't go to DL even when there is no church school.