As it is a cross-sectional study, in my opinion all questionnaires (both types) can be used, you can even ask about the old attitudes and food practices in the statistical analysis you can find associations between these patterns and obese children, overweight or normal.

If your purpose is to describe the food and/or nutrient intakes of this specific population, then generate a priori or a posteriori dietary patterns and observe correlations with dietary diversity, you can use indeed 24 hour recall, repeated 24 hour recalls, food diary or food frequency questionnaire.

Nevertheless, according the method you will use, we will not have the same estimation of the usual intake. For example, you may have difficulties to assess the intakes of episodically-consumed foods with only one 24 hour recall.

The choice of the dietary assessment method you will use should take into account the level of precision you want to achieve (accurate estimation of nutrient intakes, detailed food items, etc.) in order to address your research question.

Well, and of course, keep in mind the logistic dimension of each method: developing a questionnaire or a food list, entering and monitoring the data, etc.

http://dapa-toolkit.mrc.ac.uk/dietary-assessment/index.html

FFQ is based on long-term dietary habit, rather than short-term. Regardless the bias by self-reported methods by either approach (FFQ or 24-h recall) or any other, FFQ provides more stable  answers regarding long term dietary pattern which is just dietary pattern analysis aims for. It is commonly used in the research. In addition, you can also estimate the intake by the frequency of the food and portion size, instead only using the frequency. 

I think for dietary pattern you should use a questionnaire that is validated in children. But since there are none for ethnically diverse children in the US. Consider the questionnaire that is validated for Australian children in this article: http://www.ncbi.nlm.nih.gov/pubmed/19922040. You should also run two week days and one week end day 24-hour questionnaires. This project should be divided between two master's students, since collection is by two methods. There can be several publications generated using both 24-hour recall and a FFQ validated in a different population.

Dietary patterns can be created from a questionnaire because usual intake over the course of a year is considered. Frequency of amount (portion size) is easily obtained and such patterns as high fat or high fruits and vegetables can be determined.

For tracking individual dietary patterns, FFQ's are more appropriate. 24 hour recalls are more appropriate if you are trying to gauge a specific demographic's eating patterns.

There is a growing debate over the validity and value of data derived from memory-based dietary assessment methods (M-BMs) such as FFQs and 24HRs. My empirically supported position is that all forms of M-BMs are pseudo-scientific, produce physiologically implausible data, and should not be used in nutrition research.

Please see the recent 'Controversy & Debate' Series in the Journal of Clinical Epidemiology for details:

Archer E, Marlow ML, Lavie CJ. The Fatal Flaws of Food Frequency Questionnaires and Other Memory-Based Dietary Assessment Methods. Journal of Clinical Epidemiology. 2018.

Link: Article Controversy and Debate: Memory based Methods Paper 1: The Fa...

Archer E, Marlow ML, Lavie CJ. Controversy and Debate: Memory-Based Dietary Assessment Methods Paper #3. Journal of Clinical Epidemiology. 2018.

Link: Article Controversy and Debate: Memory-Based Dietary Assessment Meth...

For such a masters's dissertation , it will be nice to combine a food frequency questionnaire to capture dietary pattern because the result of a 24-hour diet recall will be limited.

Will the questionnaire include open ended questions so people can be specific about the foods they eat while asking more general questions about food groups they eat? The more specific foods would probably have to be coded somehow to do the analysis. Are you going to have any ranking questions, like in the food pyramid or food plate diagram, so you know which foods are eaten mostly, or course your ranking could have ranking within them? Which do eat most rank each group, then from the groups rank specific food sub-groups like dairy is a group, then a sub-group would be milk, cheese.

By 7 days food frequency questionnaire, you can calculate food consumption score. This is the best method.

https://www.researchgate.net/publication/338455379_Food_security_status_of_adolescent_girls_in_selected_urban_slum