An inexpensive pool test kit with Phenol Red will be yellow at pH < 6.8 and red at pH > 7.4. This makes it a great indicator for detecting when a titration reaches pH near 7.0, e.g. neutral. The least expensive pool test kits do two tests: free chlorine and pH and come with two reagents, one is phenol red for the pH test. Best to buy the kit from a reputable local pool store, one that won't sell you a year old kit as the reagents shelf life is affected by heat and light.
You can do 1:10, 1:100, 1:1000, and 1:10000 dillutions of your acid with distilled water to measure lower pH values using a drop of Phenol Red in the pool test kit and compare the color with the yellow pH color comparisons. pH is a log10 scale so a 1:10 dillution of an acid will measure 1.0 greater than the original solution, 1:100 will measure 2.0 greater, 1:1000 3.0 greater, 1:10000 4.0 greater. This will allow you to fairly accurately measure strong acid pH's below the point where phenol red turns bright yellow.
When dilluting acids, always add the acid to water. With very strong acids, always wear face protection, gloves, and wate/acid proof apron. BEWARE: the dilution process is exothermic - e.g. solution will get VERY HOT when using very strong acids so be sure that the container is securely held by something that can withstand the heat.
Pure lemon juice is around pH 2 which is about the lowest pH that is appropriate for experimentation with in the K-5 grade level. I would expect 1:10 and 1:100 dilutions of lemon juice to be measureable using phenol red, just subtract 1.0 for a 1:10 and subtract 2.0 for a 1:100 dillution to know what the original solution's pH was.
BEWARE: Acid base reactions can be violent. Baking soda acid reactions are going to give off a lot of gas products and they can be VERY messy. These reactions are often use to propel toy boats and rockets.