Nothing wrong with junior classes, I teach one myself. I think they are very worthwhile, can be challenging, and can be very trying at times, but ultimately rewarding
. My class has been running for a year now with 14 kids ageing between 7 and 14. By mixed classes I take it you mean junior and adult? I would not reccomend that you do this, at least not on the same mat/ training area at the same time. This is more from a safety aspect, you can imagine the damage to a child if an adult tumbles into them or a heel catches them in the face/ head during ukemi!
I notice that you`re in the UK, is your organisation part of the BAB? the BAB have guidelines and recommendations for teaching kids, you should look into these, basically give a lot of information on best practice etc. as it can be quite a minefield here in the UK. There`s a lot of info to consider, the BAB childrens coaching course is very worthwhile, if only to make you aware. A couple of clubs on my course decided to not teach children as a result of the info they got. Personally, I think that`s a bit too much, the kids are the future of aikido but you do need to ensure that all precautions are taken to protect both the kids and the club/ sensei.