There are several reasons why children are at an increased risk of developing HIV drug resistance. Some of these are to do with their dependence on the adults around them, others are to do with the treatment options themselves.
Because children rely on their caregivers to help them adhere to treatment, they are more vulnerable to experiencing treatment failure. Many caregivers will have HIV themselves, and may have their own difficulties with adherence. This can make managing a child's medication more challenging, especially if their child has to take their treatment at different times of the day and different frequencies. Where paediatric care is separate from adult HIV care, parents and caregivers may have to attend multiple appointments, meaning more travel to and from clinics and more strain on them.
Making sure that children get the right treatment to suppress HIV can also be more complicated. For example getting their dosage correct is more difficult as you need to make sure that the medication is strong enough to control the virus, but not too strong as to have bad side effects. As children grow, the dosage they need will change, increasing to match their new body weights. If their dose is too low they are at risk of developing drug resistance.
Because children start treatment at a younger age, they will be taking treatment for more time. The longer you take treatment for the more likely you are to develop drug resistance. But good adherence can control this risk.

Unfortunately there are fewer treatment options available to children. One of the reasons for this is that most new treatments are tested on adults first. It's only after the drugs have been seen to work on adults that researchers will start to consider developing paediatric versions. This means that most paediatric treatments are a few years behind what's available for adults. Children may also require drugs to be reformulated so that they taste better and come in smaller pills or pellets.
Part of the reason why adults are used to test medicines first is that it's easier to understand the effects of treatment on an adult body. Children's bodies are always changing and growing, so it's more difficult to understand the effects of treatment. There are also more ethical guidelines and rules around how to involve children in trialling medicines, as this can expose them to some risks.
Our success in being able to prevent mother-to-child transmission, during pregnancy, birth and breastfeeding, has meant that the number of children being born with HIV is shrinking. This is good news for the health of the children, but means that the market for paediatric antiretroviral drugs is also becoming smaller. As a result, companies are put off developing drugs for children living with HIV. Developing new treatments is expensive and drug companies want to know that they will be able to sell the treatments once they are ready, to make back some of their costs. This is less certain if there will be less children needing treatment in the future.