You can select all of those inputs with querySelectorAll:
const inputs = document.querySelectorAll("input.leaflet-control-layers-selector");
That gives you a NodeList, which you can loop through via forEach (in modern browsers) or index into via length and [].
For instance, adding a button:
const list = document.querySelectorAll("input.leaflet-control-layers-selector");
for (const input of list) {
input.insertAdjacentHTML(
"afterend",
"<button type=button>Some Button</button>"
);
}
<div class="leaflet-control-layers-base">
<label>
<div><input type="radio" class="leaflet-control-layers-selector" name="leaflet-base-layers_157"><span> All Activities</span></div>
</label>
<label>
<div><input type="radio" class="leaflet-control-layers-selector" name="leaflet-base-layers_157"><span> <span id="icons1">Beaches</span></span></div>
</label>
<label>
<div><input type="radio" class="leaflet-control-layers-selector" name="leaflet-base-layers_157"><span> <span id="icons2">Wading pools</span></span></div>
</label>
<label>
<div><input type="radio" class="leaflet-control-layers-selector" name="leaflet-base-layers_157"><span> <span id="icons3">Swimming pools</span></span></div>
</label>
</div>
For slightly older browsers, you may need to use forEach instead, and on even older ones, you may need to polyfill forEach. This answer goes into that part of things in detail.
Or if you wanted the buttons at the end of the spans:
const list = document.querySelectorAll("input.leaflet-control-layers-selector");
for (const input of list) {
input.nextElementSibling.insertAdjacentHTML(
"afterend",
"<button type=button>Some Button</button>"
);
}
<div class="leaflet-control-layers-base">
<label>
<div><input type="radio" class="leaflet-control-layers-selector" name="leaflet-base-layers_157"><span> All Activities</span></div>
</label>
<label>
<div><input type="radio" class="leaflet-control-layers-selector" name="leaflet-base-layers_157"><span> <span id="icons1">Beaches</span></span></div>
</label>
<label>
<div><input type="radio" class="leaflet-control-layers-selector" name="leaflet-base-layers_157"><span> <span id="icons2">Wading pools</span></span></div>
</label>
<label>
<div><input type="radio" class="leaflet-control-layers-selector" name="leaflet-base-layers_157"><span> <span id="icons3">Swimming pools</span></span></div>
</label>
</div>
Etc.