mammy,
DS (7) has recently started Duolingo (French) and I agree reception age is probably too young. It is aimed at adults, really. As far as I can make out it doesn't 'teach' as such but introduces new vocab gradually: First you need to find the picture that shows the new word (e.g. 'the men'), and the word in French is read out to you (les hommes). Then later in the same session you need to recall that word in order to complete the exercises. Each session introduces a few words, some will be practised by giving the French word and you having to translate into English, some the other way. So for example "Translate to English: Les hommes" or "Write in French: The men". Some words are not initially introduced by picture, but 'mouse over' the 'red' word (I think words that you haven't been introduced yet are shown in red) will show you the solution, and this word will usually be queried again later in the session.
The actual grammar can be picked up by osmosis during the sessions, or you have to read the lesson notes. So e.g. you can learn by repetition and pattern detection that plurals come with 's' and that adjectives are adjusted in number/gender to the nouns they are describing, so come with 's' also. But this isn't 'taught' explicitly. So if you don't pick up the pattern automatically, you have to study the notes, which amounts to about the same as reading any textbook, clearly not appropriate to 4/5 year olds. Unless you as the parent use the notes as a teaching guide and teach your child.
Seeing as DS is quite good at picking up patterns without explicit instruction, I let him get on with it and (knowing French quite well myself) occasionally point out to him what he is supposed to be learning here. But at 7, we are consciously doing this in parallel to weekly French lessons at school and attending a further French Club (also at school, but paid for) because I believe children especially need to learn languages through hearing and using them orally.
I have come across an app called 'mes premiers mots' (currently free on kindle fire for kids unlimited), but cannot say anything about how good/bad it might be.