Thank you for your question, it’s great that you are reaching out for more support on this one. Horsetail is okay to give to children as a tea, yes. It can be a great support for bet wetting. It does have high silica content and a compound that breaks down Vit B1, so not taken at too high a dose, or for more than a couple of weeks and boiled for a few minutes at least while making tea. Dose for a child of 7 (as long as they are within average weight and health-e.g. no liver and kidney concerns) starting at 1/2tsp dried/cup, 3x/day.
The other herbs you mention may also help greatly, but an important starting point for creating an effective formula is to look into the potential causes behind the bed wetting (nocturnal enuresis) if it is over 2 times a week at age 7, or if it has started up when previously your child has had dry nights for a while. It can be completely normal, but it’s also important to make sure all is well as either of these situations could be an indicator that there is something going on, such as an urinary tract infection, stress (is there something happening at school or at home that may be causing more than usual), differences in the structure of their bladder, sphincter or pelvic floor or unfortunately it can be a red flag for abuse (I’ll highlight that of course this may not be the case as well, but it is important to mention it as a potential so that it does not get overlooked).
Herbs can be of great help all around in many of these situations, but the formula created will be much more effective once some more background information is reviewed. It will likely be of support to check in with a Registered Herbalist or student at the Pacific Rim College Supervised Student Clinic: https://pacificrimcollege.janeapp.com/