Multi-vitamins are a waste of money in most cases. They even have warning labels on them - keep out of reach of children! Yes, they cause constipation in many kids. Don't know why your pedi recommended them since most doctors will say that they are just making "expensive urine" since most of the nutrition is eliminated. Frankly, you don't know anything about those vitamins - many U.S. companies have outsourced their production to other countries and the pills come in contact with many hands. Also, different forms of wax and even shellac are used to get the pills to pop out of their molds during manufacture. If you want to give me your email address, I can send you some info from the Physicians Desk Reference to document this.
Most doctors did not take even one nutrition course in medical school so they really don't have a lot of info on this subject - so they often don't make the best recommendations!
Also, vitamins are made to be taken in conjunction with other vitamins, minerals, trace elements, phyto-nutrients (plant based), amino acids, etc. Taking something (e.g. vitamin B) in isolation or in an unbalanced formula can make it virtually useless - it's not going to work without the other components/partner elements it needs.
I am a nutritional consultant and I have worked with many, many people who have constipation and constipated children. There is a safe supplement you can use that will eliminate this problem and I would be happy to help you with it. It's not a pill - it's a delicious vanilla or chocolate powdered supplement that mixes with any liquid she's drinking. It's used all over the world with severely compromised kids as well as by healthy kids here to keep them healthy. I have no doubt that it will help strengthen her nails as many people have reported success with this as well. I'd be happy to share more info with you.