So sorry about your little girl's fissure. Witch Hazel is known to offer topical relief for the pain & works wonderfully to reduce inflammation. It's also great as a compress after childbirth! You can pick it up at any grocery store, usually by the pharmacy & it's around $1. Just saturate a cotton ball or cloth & pat/wipe softly. It's cold, but feels good when applied to a sore, inflamed area.
you're probably looking at her diet to see if there are constipation triggers, but keep in mind that it's VERY common at this age for kids to hold their poop & she might not even have allergies. Toddlers become very aware of their bodies & are trying to figure out how things work. There are side effects to all medication, even Miralax. Some very effective things that are healthful, can be used daily without side effects & can also be used in conjunction with medications if needed, are probiotics & also Omega fatty acids.
I swear by these & they are staples with us & our children. The yogurts with the live strains in them are okay as well, but contain alot of sugar & when you're having constipation issues sugar makes it worse. So does the dairy. A very reputable children's brand is Nature's Plus "Acidophikidz". It's a chewable tablet that my kids love & they're pretty picky.
Spectrum Essentials "children's DHA Omega 3 for Brain & Vision Development" is an excellent way to keep you child's intestinal tract healthy & moving things right along, even though it's labeled to promote vision/brain devel. This product contains 500mg Flax Seed Oil,which is hard to get down otherwise due to the taste & why I love this children's version. They are tiny little chewable gel caps that are naturally flavored with strawberry/banana & contain no sugar. Flax Seed Oil offers a ton of healthful benefits & is something you can use as a daily supplement.
And the most important thing is to of course trust your instinct. Testing can be very invasive & many dr's are quick to write a Rx & order a test to rule out the worst, but make sure you're comfortable with whatever course you take. We've been through quite a lot with both our daughters & the medical system. Every time I trust the dr's over my own judgement, I regret it.